xdg-shell: handle clients launched in fullscreen

xdg-shell: handle clients launched in fullscreen

When client is lauched in fullscreen, it is placed on the
first output, because it is not mapped and
shell_surface_set_output() therefore sets default output.

Since we have no better way how to position newly created windows,
(http://lists.freedesktop.org/archives/wayland-devel/2014-May/thread.html#14568)
set the output to the one that has currently focus. Priority has
the touch focus, then pointer and then keyboard focus.

This fixes bug
https://bugs.freedesktop.org/show_bug.cgi?id=69780

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>