Allow fallback for input method modules

Accept a :-separated list of module names in GTK_IM_MODULE and
the corresponding setting, to deal a bit better with broken
situations.

https://bugzilla.gnome.org/show_bug.cgi?id=603559

gtk/gtksettings.c

@@ -947,6 +947,8 @@ gtk_settings_class_init (GtkSettingsClass *class)
    * Which IM (input method) module should be used by default. This is the
    * input method that will be used if the user has not explicitly chosen
    * another input method from the IM context menu.
+   * This also can be a colon-separated list of input methods, which GTK+
+   * will try in turn until it finds one available on the system.
    *
    * See #GtkIMContext and see the #GtkSettings:gtk-show-input-method-menu property.
    */