Add support for WM_CLIENT_LEADER, put floating windows mapping to (0x0) to center of leader/workspace

include/data.h

@@ -341,6 +341,10 @@ struct Client {
         /** Holds the WM_CLASS, useful for matching the client in commands */
         char *window_class;
 
+        /** Holds the xcb_window_t (just an ID) for the leader window (logical
+         * parent for toolwindows and similar floating windows) */
+        xcb_window_t leader;
+
         /** fullscreen is pretty obvious */
         bool fullscreen;