add overview based on kwin's placement algorithm

kwin/src/plugins/private/expolayout.cpp

kwin's overview placement algorithm first sorts by window height(2nd key center_y),

turning it into a one-dimensional row partitioning problem.

(limit binary search iteration to guarantee termination)

Then it sorts within the rows based on window center_x.

(first sort fallback rev creation_id
for stablility)

include/labwc.h

@@ -20,6 +20,7 @@ enum input_mode {
 	LAB_INPUT_STATE_RESIZE,
 	LAB_INPUT_STATE_MENU,
 	LAB_INPUT_STATE_CYCLE, /* a.k.a. window switching */
+	LAB_INPUT_STATE_OVERVIEW, /* overview mode */
 };
 
 struct seat {