Undo/redo function in the rooms editor

I've added undo/redo buttons in the rooms editor. The undo is available when the user, with the mouse or with text fields, adds, deletes and moves an object or a tile. The undo and redo can be used also with the shortcut keys : ctrl-z for undo and ctrl-y for redo.  The limit of the undo history is 100. If a sprite, a background or an object is modified in the left tree, the undo is reset.