Add screenshot capability

Currently, when pressing PrintScn (like on Windows) or Alt-Shift-3 (like on the Mac), then nothing happens.

`/usr/bin/lximage-qt --screenshot` seems like it could be a good starting point.

Perhaps some code from https://github.com/lxqt/lximage-qt can be re-used.

Especially for development, screenshot capability is an important feature ;-)