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/theme.h

@@ -165,6 +165,8 @@ struct theme {
 	float osd_border_color[4];
 	float osd_label_text_color[4];
 
+	float overview_bg_color[4];
+
 	struct window_switcher_classic_theme {
 		int width;
 		int padding;