keyboard: Apply XKB layouts ourselves and stop relying on libgnomekbd

libgnomekbd/xklavier aren't a good fit to have the keyboard input
story that we want since they rely on implementation details of the
XKB protocol to provide users with a means to switch keyboard layouts.

Of note here is a) their reliance on XKB groups, of which there can be
only up to 4, to specify the layouts the user is able to switch
between and b) their reliance on XKB options to specify the keybinding
to switch layouts which is a restricted set and falls outside the
regular GNOME desktop wide keybindings management as it's implemented
entirely on the X server side.

This commit introduces the use of a shared GSettings schema from
gsettings-desktop-schemas which will be the storage for our new
concept of "input sources".

We only handle XKB layouts as input sources for now. We do it using
roughly the same method that setxkbmap(1) uses which should allow the
users that want to specify their own XKB features (except layout) to
still do so outside of GNOME (e.g. in a session startup script).

https://bugzilla.gnome.org/show_bug.cgi?id=676102