client: Add mediated reader stopping for display proxy

Add display API for negotiated display event reader's early exits.

Closes #28

This patch allows display event readers to exit their polling and
processing loops early based on when something calls wl_display_stop.

TODO: There is still a potential issue of new threads starting
after wl_display_stop has returned, which would need a more
significant change to give the display API control of the
display pointer, and check whether it is nulled before dereferencing
throughout the current API.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
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