Only warp pointer once during x_push_changes()

- Introduce warp_to static variable in x.c that stores the coordinates
  to warp to as a Rect.

- Add x_set_warp_to function to set this variable.  Use in _tree_next,
  workspace_show, and con_move_to_workspace.

- In x_push_chanages, if warp_to is set, then call xcb_warp_pointer_rect
  and then reset it to NULL.

This fixes all know bugs for pointer warping for me.

src/tree.c

@@ -414,8 +414,10 @@ static bool _tree_next(Con *con, char way, orientation_t orientation, bool wrap)
 
         workspace_show(workspace->name);
         Con *focus = con_descend_direction(workspace, direction);
-        if (focus)
+        if (focus) {
             con_focus(focus);
+            x_set_warp_to(&(focus->rect));
+        }
         return true;
     }