Extwindow: Fixed crash when changing stay-on-top key.
Prefs: Simplified code using GSettings bindings for toggles.

We do not need to take care anymore of:
- sensitivity of the widget wrt the writable status of the key
- modification of the widget when the key is updated
- modification of the key when the widget is updated
- preventing infinite loops
Prefs: Removed useless gm_prefs_entry_new parameters.
Prefs: Simplified code using GSettings bindings for GTK entries.
Prefs: Ported GTK adjustments to GSettings bindings to simplify code.
Prefs: Ported string option menu to GSettings binding.

Also simplified code by using a GtkComboBoxText instead of a complex
GtkComboBox with a GtkTreeModel.
Prefs: Ported int_option_menu to GSettings bindings.