layer: improve keyboard-interactivity

layer: improve keyboard-interactivity

- Process layer-shell keyboard interactivity in the map and commit
  handlers only, rather than in layers_arrange(). This allows handling of
  the layer-surface that emitted the event rather than iterating over all
  surfaces in the output layer-tree, and therefore avoids having to guess
  which surface should have keyboard preference (and it might not be the
  last one added to the list which was the assumption previously).

- Prevent seat_focus_surface() from setting keyboard focus if a layer-shell
  surface with exclusive keyboard-interactivity has the focus.

- Set cursor_context type for layer-surfaces without node-descriptors
  in order to set keyboard focus correctly in cursor_button_press().

Tested satisfactorily with xfce4-panel and gtk-layer-demo.

Fixes #725 and #704