Add option to fullscreen during breaks

It may help some users' workflows if the timer is fullscreened during breaks to remind the user to do something not on their computer. Not all users would want this, so it should be an option and off by default. It unfullscreens when the work time resumes.

I think it makes more sense to not fullscreen and unfullscreen until the timer for the next phase is started, but I'd be open to feedback on that.

Built and tested on amd64/x86_64.

Closes #61 (closed)