Refactor wayland focus management
This MR is a chunk of !3420 (merged), split to ease review.
Along with some small cleanups, these commits change Wayland keyboard focus management (& co, tablet pads, IM, clipboard) so it gets hooked through the MetaDisplay::focus-window signal, like the X11 bits started doing at !3269 (merged). This makes MetaDisplay consistently more of a "model" in this regard, with the wayland/x11 paths reacting separately in consequence.