server: stop wl_display event loop from any context

Calling wl_display_terminate() will exit the wl_display event loop
at the start of the next loop iteration.  This works fine when
wl_display_terminate() is called after the event loop wakes up
from polling on the added event sources.  If, however, it is
called before polling starts, the event loop will not exit until
one or more event sources trigger.  Depending on the types of event
sources, they may never trigger (or may not trigger for a long time),
so the event loop may never exit.

Add an extra event source to the wl_display event loop that will trigger
whenever wl_display_terminate() is called, so that the event loop will
always exit.

Fixes #201

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>