Implement floating (please test and find bugs)

Details which are missing: A command to hide/show all floating clients,
moving/resizing clients with your mouse holding Mod1 (click anywhere
in the client, not just on its borders), resize/move by keyboard, select
next/previous client by keyboard

include/data.h

@@ -253,6 +253,8 @@ struct Client {
 
         /* x, y, width, height of the frame */
         Rect rect;
+        /* Position in floating mode and in tiling mode are saved separately */
+        Rect floating_rect;
         /* x, y, width, height of the child (relative to its frame) */
         Rect child_rect;
 
@@ -282,6 +284,9 @@ struct Client {
         /* fullscreen is pretty obvious */
         bool fullscreen;
 
+        /* floating? (= not in tiling layout) */
+        bool floating;
+
         /* Ensure TITLEBAR_TOP maps to 0 because we use calloc for initialization later */
         enum { TITLEBAR_TOP = 0, TITLEBAR_LEFT, TITLEBAR_RIGHT, TITLEBAR_BOTTOM, TITLEBAR_OFF } titlebar_position;