kiosk-shell: Embed keyboard focus activation code

Just like desktop-shell, we shouldn't be dependent on having a keyboard
be present in order to activate a window/surface.

This adds a libweston helper to retrieve the first available seat, and
to use it in order to avoid going over the seat list.

We also encapsulate the activation of the surface in one place, and use
it on surface removal, when the surface has been committed, or for
touch/pointer events. With it we also deal with the keyboard focus and
shell activation in one place.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>