kiosk-shell: Introduce kiosk/fullscreen shell for desktop apps

kiosk-shell: Introduce kiosk/fullscreen shell for desktop apps

kiosk-shell is fullscreen shell for apps that use the xdg-shell
protocol. The goal is to make life easier for people shipping embedded
devices with simple fullscreen shell requirements, and reduce the
proliferation of desktop-shell hacks.

Top level surfaces are made fullscreen, whereas dialogs are placed on
top in the center of the output and retain their natural sizes. Dialogs
can be moved and (un)maximized, but resizing is currently not supported.

An app can be directed to a particular output by populating the
"app-ids" field with the app's XDG app id, in the relevant
"[output]" section in the weston config file.

Fixes: #277
Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>