Ported GtkEntry widgets of the prefs window to GSettings.

We also now use Ekiga::Settings despite it is C code. It is at least useful
to handle shared_ptr and good memory deallocation.

Corresponding methods have been removed from the gmconf* files.
C++ised more things.
Fixed flickering effect when windows are shown.

This was due to the fact the window was first shown, then moved to the
last saved position. This seems to be "new" with GTK3. We can now move
the window to the correct position when the realize signal is emitted.

That prevents the flickering effect. However, it requires the GmWindow
to be initialized from the start with the correct GSettings key. The
call window and the main window were the only one with a differed key
setup. This is now fixed too.

The former gm_window_set_key methods have been removed as they can not
be used anymore.
GSettings: Ported toggles to GSettings.
GSettings: Ported remaining int_option_menu to GSettings.
GSettings: Completely ported PrefsWindow to GSettings.

Also got rid of old gmconfwidgets.* and gmpreferences.* files. All methods
have been moved as static methods into the preferences-window.cpp file itself.
GmMenuAddon: Ported TOGGLE_ENTRY and RADIO_MENU_ENTRY to GSettings.

I don't like having GSettings things in the callbacks, but that is a pure
port of the old code. Things can be improved at a later stage if required.
Ported the Call Window and the Main Window to GSettings.
Ported the Call Window and the Main Window to GSettings.
MainWindow: Fixed crash due to signal loop.
MainWindow: Fixed signal loop.
GmWindow: Completed GSettings porting.

There was a forgotten gmconf function.
LDAP: Ported to GSettings.
Fixed the gstreamer plugin compilation with GSettings
Added an experimental Opal::Presentity implementation

This is heavily based on Local::Presentity's code, but with some little
improvements here and there (pushing build_node as a static method makes
a better api, for instance), removed presence fetching/unfetching code
(which will need to be re-added later, perhaps differently).

It hasn't been added to the build system, and was never compiled.
Opal::Presentity: I see no need for that doc parameter!
Opal::Presentity: try to get rid of the need for the whole cluster...
Opal::Presentity: static is only for the declaration in the class
Opal::Bank: first draft of the modification
Opal::Account: first draft