core: delay focus updates

Delay focus updates until critical section. Rational is that focus
events might arrive when the focused window hasn't been managed by
compton, result in that that window not being focused.

This commit makes compton mark focus update events, and only update
focus in critical section, after we managed all the new windows.

Fixes #177

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/common.h

@@ -230,6 +230,8 @@ typedef struct session {
 	/// If compton should quit
 	bool quit:1;
 	/// Whether there are pending updates, like window creation, etc.
+	/// TODO use separate flags for dfferent kinds of updates so we don't
+	/// waste our time.
 	bool pending_updates:1;
 
 	// === Expose event related ===