Adding cut, copy and paste functions in the rooms editor

Three new buttons have been added in the rooms editor : cut, copy and paste. They can be called with the standard keyboard shortcuts : Ctrl+X, Ctrl+C and Ctrl+V. These functions can be used with the new selection tool.To give an example, let's say that you want to copy a region in the room. First, you click on the selection tool and select the region to copy. Then you do an ctrl-c (copy) and a ctrl-v (paste). You have now a stamp, and every time you click on the left button, the instances are pasted in the room (c.f http://enigma-dev.org/forums/index.php?topic=2082.105).
LateralGM has now three different states in the rooms editor : single object selection (the default), region selection and a pasting mode. I added a new toggle button in the toolbar in order to activate the single object selection. This is needed when we are in pasting mode and we want to go back in single object selection.