Port to GSettings, GAction and PanelApplet 5.0

Upstream gnome-panel recently released version 3.14 with some API changes:

* `Applet.get_preferences_key()` was removed, which means applets should use GSettings instead of deprecated gconf.
* Deprecated `Gtk.Action` was replaced with `Gio.SimpleAction`.

This PR does the necessary changes to work with new API. With first commit, it will still work with old gnome-panel, but after applying second commit, it will work only with new gnome-panel (which is in Debian NEW queue, and will be soon available in experimental).