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.

include/wx/private/secretstore.h

@@ -79,14 +79,14 @@ class wxSecretStoreImpl : public wxRefCounter
 {
 public:
     virtual bool Save(const wxString& service,
-                      const wxString& user,
-                      const wxSecretValueImpl& secret,
+                      const wxString& username,
+                      const wxSecretValueImpl& password,
                       wxString& errmsg) = 0;
-    virtual wxSecretValueImpl* Load(const wxString& service,
-                                    const wxString& user,
-                                    wxString& errmsg) const = 0;
+    virtual bool Load(const wxString& service,
+                      wxString* username,
+                      wxSecretValueImpl** password,
+                      wxString& errmsg) const = 0;
     virtual bool Delete(const wxString& service,
-                        const wxString& user,
                         wxString& errmsg) = 0;
 };