Unify keymap-reloading code branches

Simplify the keymap loading logic by unifying the different
branches; in the reorganization this patch fixes a bug where when
we got a MappingKeyboard event we wouldn't update virtual modifiers
correctly.

Based on a patch by Thomas Thurman <tthurman@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=565540