Add FCmdrSettingsBackend.

Abstract base class for extensions that handle different types of
settings in a profile, such as GSettings or app-specific settings.

Also establish a central place to register extension points and
built-in extensions for the system service.
Add FCmdrGSettingsBackend.

Just a stub for now to test the settings backend loading mechanism.

Each member name in the "settings" JSON object will double as the
FCmdrSettingsBackend extension name.  So "org.gnome.gsettings" in
the JSON data will instantiate an FCmdrGSettingsBackend by way of:

  g_io_extension_point_implement (
          FCMDR_SETTINGS_BACKEND_EXTENSION_POINT_NAME,
          g_define_type_id,
          "org.gnome.gsettings", 0);