Change wxSecretStore API to allow retrieving the username

The old API didn't make any sense for the most common case when both the
user name and password need to be stored, as it required providing the
user name as input, which couldn't work (but somehow this went
unnoticed for more than a year...).

Fix this by returning the username, and not only the password, from
Load() instead of taking it as parameter and removing this parameter
from Delete() as well.

Also improve the documentation, notably include a simple example of
using this class.

Notice that this is a backwards-incompatible change, but the old API was
really badly broken and didn't appear in 3.1.0 yet, so the breakage is
both unavoidable and, hopefully, shouldn't affect much code.
Nevertheless, a special wxHAS_SECRETSTORE_LOAD_USERNAME symbol is added
to allow testing for it if necessary.

src/unix/secretstore.cpp

@@ -33,6 +33,8 @@
 #include <libsecret/secret.h>
 
 #include "wx/gtk/private/error.h"
+#include "wx/gtk/private/list.h"
+#include "wx/gtk/private/object.h"
 
 namespace
 {
@@ -131,10 +133,9 @@ public:
                       wxString& errmsg) wxOVERRIDE
     {
         // We don't have any argument for the user-visible secret description
-        // supported by libsecret, so we just concatenate the service and user
-        // strings. It might be a good idea to add a possibility to specify a
-        // more informative description later.
-        const wxString label = service + wxS("/") + user;
+        // supported by libsecret, so we just reuse the service string. It
+        // might be a good idea to add a possibility to specify a more
+        // informative description later.
 
         // Notice that we can't use secret_password_store_sync() here because
         // our secret can contain NULs, so we must pass by the lower level API.
@@ -145,7 +146,7 @@ public:
                 GetSchema(),
                 BuildAttributes(service, user),
                 SECRET_COLLECTION_DEFAULT,
-                label.utf8_str(),
+                service.utf8_str(),
                 static_cast<const wxSecretValueLibSecretImpl&>(secret).GetValue(),
                 NULL,                           // Can't be cancelled
                 error.Out()
@@ -158,21 +159,27 @@ public:
         return true;
     }
 
-    virtual wxSecretValueImpl* Load(const wxString& service,
-                                    const wxString& user,
-                                    wxString& errmsg) const wxOVERRIDE
+    virtual bool Load(const wxString& service,
+                      wxString* user,
+                      wxSecretValueImpl** secret,
+                      wxString& errmsg) const wxOVERRIDE
     {
         wxGtkError error;
-        SecretValue* const value = secret_service_lookup_sync
-              (
+        GList* const found = secret_service_search_sync
+            (
                 NULL,                           // Default service
                 GetSchema(),
-                BuildAttributes(service, user),
+                BuildAttributes(service),
+                static_cast<SecretSearchFlags>
+                (
+                    SECRET_SEARCH_UNLOCK |
+                    SECRET_SEARCH_LOAD_SECRETS
+                ),
                 NULL,                           // Can't be cancelled
                 error.Out()
-              );
+            );
 
-        if ( !value )
+        if ( !found )
         {
             // There can be no error message if the secret was just not found
             // and no other error occurred -- just leave the error message
@@ -180,21 +187,32 @@ public:
             // behave.
             if ( error )
                 errmsg = error.GetMessage();
-            return NULL;
+            return false;
         }
 
-        return new wxSecretValueLibSecretImpl(value);
+        wxGtkList ensureListFreed(found);
+
+        SecretItem* const item = static_cast<SecretItem*>(found->data);
+        wxGtkObject<SecretItem> ensureItemFreed(item);
+
+        const wxGHashTable attrs(secret_item_get_attributes(item));
+        const gpointer field = g_hash_table_lookup(attrs, FIELD_USER);
+        if ( field )
+            *user = wxString::FromUTF8(static_cast<char*>(field));
+
+        *secret = new wxSecretValueLibSecretImpl(secret_item_get_secret(item));
+
+        return true;
     }
 
     virtual bool Delete(const wxString& service,
-                        const wxString& user,
                         wxString& errmsg) wxOVERRIDE
     {
         wxGtkError error;
         if ( !secret_password_clearv_sync
               (
                 GetSchema(),
-                BuildAttributes(service, user),
+                BuildAttributes(service),
                 NULL,                           // Can't be cancelled
                 error.Out()
               ) )
@@ -232,8 +250,18 @@ private:
     }
 
     // Return attributes for the schema defined above.
+    static wxGHashTable BuildAttributes(const wxString& service)
+    {
+        return wxGHashTable(secret_attributes_build
+                            (
+                                GetSchema(),
+                                FIELD_SERVICE,  service.utf8_str().data(),
+                                NULL
+                            ));
+    }
+
     static wxGHashTable BuildAttributes(const wxString& service,
-                                       const wxString& user)
+                                        const wxString& user)
     {
         return wxGHashTable(secret_attributes_build
                             (