Revision 58

	* Thuban/UI/mainwindow.py (MainWindow.NewSession): Use
	create_empty_session session to create the new, empty session.

	* Thuban/UI/mainwindow.py (MainWindow.__init__): Set the size of
	the tool bitmaps.
	(MainWindow.OnClose, MainWindow.save_modified_session): Separate
	the code that asks whether the session should be saved into the
	new method save_modified_session.
	(MainWindow.OpenSession, MainWindow.NewSession): Use the new
	method to save modified session here too.