Adding rectangular selection tool

A new rectangular selection button has been added in the rooms editor toolbar. When the button is pressed, the user can draw a rectangular selection in the room. By pressing the delete key, or by clicking on the delete button, the instances/tiles are deleted. Next to the selection button, the fill button allows to fill the region with a tile or an instance. If the 'snap to grid' mode is activated, the region is filled according to the grid's cell, otherwise it will take into account the tile/instance width. I added also three new buttons : 'Snap to grid', 'Add on top' and 'Add multiple'. These buttons are used to switch the corresponding modes.
For future releases, it would be nice to have : undo for the new functions, and automatic scrolling when the user draw a region which goes beyond the viewport.
In my next pull request, i will implement the copy and paste functions for the selection tool.