screencast/region: add persistent overlay once screencasting begun

Apparently some programs think it's funny to not show a preview of what you're actually screensharing. Pretty annoying if you're sharing a region, and you don't know what contents are still visible to your viewers.

This MR remedies that by adding an overlay that is similar to that of spectacle, albeit with a straight line and not a dashed one, in order to avoid confusion.

Draft for now until I test multi-monitor (or get someone to do it for me :) ) multi monitor works for me!