input: fix keyboard focus crash

If two or more clients were running and the one that was focused when
weston itself lost keyboard focus was killed, weston would crash.

This is because commit 85d55540 changed the way we handle saved keyboard
focus when we lose focus, and did so in such a way that the saved keyboard
focus listener could be removed from the surface destroy signal list
during the emit of the surface destroy signal. This corrupted the list
and led to a NULL pointer dereference.

Fix this by using a boolean flag to determine whether we should obey the
saved keyboard focus. We can set this safely in cases where
removing the listener would cause a crash.

Fixes #138

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>