backends: Add methods to handle keymaps

These methods allow us to set and get xkbcommon keymaps as well as
locking a specific layout in a layout group.

With this, we introduce dependencies on xkeyboard-config, xkbfile,
xkbcommon-x11 and a libX11 new enough to have xcb support.

https://bugzilla.gnome.org/show_bug.cgi?id=734301