Enable spare keyboard layouts in Wayland

@butirsky , @odeda , @dion and @ngraham - this is still a WIP, so please read the text below. But I would be very happy if you could find some time to test the code in this branch / MR. I will continue to update it throughout the next days, but I just want to be sure, that we're moving in the right direction.

This is the initial commit, which implements the basic functionality of spare keyboard layouts for Wayland. It works 1:1 as in X, but it still hasn't been fully tested - e.g. missing tests for different configurations for loop count and spare layouts count.

Furthermore, the tray layout indicator looses sync - it stops showing the right selected layout at some point. The OSD though shows the right layout. (see comments below)