Add wxSecretStore

Add a new class allowing to store passwords and other sensitive information
using the OS-provided facilities.

Add implementations for all the main platforms, documentation and a new sample
(which contains an ad hoc unit test as the real unit test for this class would
probably be a bad idea as it wouldn't run in non-interactive contexts and
could show OS level dialog boxes if it did).

include/wx/gtk/private/error.h

@@ -31,6 +31,12 @@ public:
         return &m_error;
     }
 
+    // Check if any error actually occurred.
+    operator bool() const
+    {
+        return m_error != NULL;
+    }
+
     wxString GetMessage() const
     {
         return wxString::FromUTF8(m_error->message);