window: add display_exit()

Add a function, that schedules the display_run() event loop to break
out.

When display_exit() is called, processing continues as usual, until
currently waiting events and deferred tasks have been processed, and
sent requests are flushed. Then, display_run() will return.

This enables toytoolkit apps to handle their exit instead of just being
killed or call exit().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/clients/window.h b/clients/window.h
index 59397f0..20bf4e2 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -135,6 +135,9 @@
 void
 display_run(struct display *d);
 
+void
+display_exit(struct display *d);
+
 enum pointer_type {
 	POINTER_BOTTOM_LEFT,
 	POINTER_BOTTOM_RIGHT,