layer-shell: implement on-demand keyboard interactivity

[alebastr]

I'm still not sure if the approach with seat_set_focus_surface is right but on-demand surface is definitely not expected to set seat->focused_layer.

Basic cases tested with gtk-layer-demo:

• xdg toplevel + layer-surface/top/on-demand:
  Input focus can be switched between the surfaces
• xdg toplevel + layer-surface/top/on-demand + layer-surface/top/exclusive:
  Focus can be switched between two layer surfaces. xdg surface does not get focus
• xdg toplevel + layer-surface/top/on-demand + layer-surface/overlay/exclusive:
  Contrary to my expectations, both layer surfaces can get focus. xdg surface is still ignored
• focus_follows_mouse: keyboard focus is released on wl_pointer.leave; keyboard focus is not received on wl_pointer.enter. The code for that is still missing.
• return keyboard focus after destroying on-demand layer surface:
  That part is definitely buggy. There's no keyboard focus after destroying on-demand layer surface.