Add checkbox to exit on mouse click.

Currently the software has already got the checkbox to exit on "mouse move".
In fact, when we move our mice the screensaver won't stop, because the default for that option is "unticked" (i.e., don't exit on mouse move).
It could be nice to also have the same checkbox for "mouse click", and maybe also for "key press" (on the keyboard).
One of the these three options should be mandatory, obviously.