desktop-shell: use weston_compositor_exit

Use the proper weston_compositor_exit API instead of wl_display_terminate() to
allow the compositor main to prepare for exit, and most importantly to set the
exit error code as appropriate.

I have some brokenness in my test suite running, and weston-desktop-shell was
crashing at start, yet the tests did not notice. With this patch, the tests
where the helper crashes are properly marked as failed.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>