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/common/node-type.h

@@ -48,6 +48,7 @@ enum lab_node_type {
 	LAB_NODE_ROOT,
 	LAB_NODE_MENUITEM,
 	LAB_NODE_CYCLE_OSD_ITEM,
+	LAB_NODE_OVERVIEW_ITEM,
 	LAB_NODE_LAYER_SURFACE,
 	LAB_NODE_UNMANAGED,
 	LAB_NODE_ALL,