libweston/backend-x11: Tracking previous events over multiple calls

Rather than doing it with a local variable, track any previous events by
hanging it out of the x11 backend and use it to handle keymap notify
events.

In this way we avoid corrupting the surface destroy signal list, in
notify_keyboard_focus_out(), ultimately leading to a crash.

Fixes #649, #650

Suggested-by: Daniel Stone <daniel.stone@collabora.com>
Reported-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>