Make it possible to select text in the GUI for copy-paste.

Some setup windows have a Python button giving a window with the Python code that gives the same result as the GUI. Unfortunately, it is not possible to copy-and-paste the Python code, that seems almost perfidious. This is because the widget does not receive focus (see http://stackoverflow.com/questions/21873195/readonly-tkinter-text-widget), this patch fixes that for all text widgets in ase-gui. I think it is safe to enable it globally.