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.

include/x.h

@@ -104,4 +104,11 @@ void x_set_name(Con *con, const char *name);
  */
 void x_set_i3_atoms();
 
+/**
+ * Set warp_to coordinates.  This will trigger on the next call to
+ * x_push_changes().
+ *
+ */
+void x_set_warp_to(Rect *rect);
+
 #endif