keyboard: Make it possible to stop watching for WM changes

As we don't want to be doing that anymore if we're not in the keyboard
panel anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=736117
keyboard: Fix a crash if the WM changes (or restarts)

By stopping watching for WM changes when leaving the shortcuts panel.

 #0 reload_sections
 #1 wm_window_event_filter
 #2 gdk_event_apply_filters at gdkeventsource.c:81
 #3 gdk_event_source_translate_event at gdkeventsource.c:195
 #4 _gdk_x11_display_queue_events at gdkeventsource.c:338
 #5 gdk_display_get_event at gdkdisplay.c:313
 #10 g_main_context_iteration at gmain.c:3766
 #11 g_application_run at gapplication.c:1623

See https://bugzilla.redhat.com/show_bug.cgi?id=1094480

https://bugzilla.gnome.org/show_bug.cgi?id=736117