I mean something like e.g. MS Paint have:
(http://i.imgur.com/HoLypMq.png)
Not only it would be more intuitive, but also it has the added benefit of being able to resize in any direction you want.
Hmmm, maybe a good idea yes, and it would also solve this issue: https://github.com/christopho/solarus-quest-editor/issues/111