Pause when there are full-screen windows

[AllanChain]

See also #3, #9.

This can be useful when doing presentations, or watching video on full-screen.

However, it can be tricky to implement because there is no unified API to detect full-screen windows for all the platforms. And it looks impossible to do so on Wayland.

Anyway, in the case of doing presentations, "pause on battery" can do the trick. For other cases, "pause on specific program running" may work too. Therefore, I'll put this in low priority.