Nested X11: use KeymapNotify events to fix key state on FocusIn

If the user Alt-Tabs out of the window, we will be left thinking
the Alt key is still pressed since we don't see a release for it.

Solve this and other related issues for the nested X11 compositor
by selecting for KeymapStateMask which causes a KeymapNotify event
to be sent after each FocusIn, and when we get these events, update
the internal XKB state and send any necessary modifiers events to
clients.

https://bugzilla.gnome.org/show_bug.cgi?id=753948