Handle touch interaction on Wayland
The desktop-icons UI happens to work on X11 with touchscreens as we get pointer emulation from the X server "for free", this does not happen however in wayland, where touch events must be handled explicitly.
Make a few touchups to generalize input handling, and handle touch-event
where it matters, so that there's complete parity between X11 and Wayland wrt touchscreens.