GtkPlug: fix handling of key events for different layouts

GtkPlug: fix handling of key events for different layouts

GtkPlug directly handles X KeyPress/Release events, instead of using
translation in GDK (which expects XI2 events for XI2). When this
was done, the handling of the group was stubbed out and never replaced.

Export gdk_keymap_x11_group_for_state() and gdk_keymap_x11_is_modifier()
so we can fill out the fields correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=675167