implement configure requests, adapt testcase

testcase does not pass 100% due to clients not being reparented correctly yet.

include/util.h

@@ -37,6 +37,7 @@ while (0)
 int min(int a, int b);
 int max(int a, int b);
 bool rect_contains(Rect rect, uint32_t x, uint32_t y);
+Rect rect_add(Rect a, Rect b);
 
 /**
  * Updates *destination with new_value and returns true if it was changed or false