Add support for screen sharing

[computerality]

Now that browsers have standardized around getDisplayMedia(), it is time to add screen sharing.
It doesn't seem like open-easyrtc has support for the proper apis yet so they will need to be patched in. It may be non-trivial to support sending audio at the same time as the screen sharing because some browsers don't allow for capturing both with a single stream.

Think it makes sense to either show a computer button next to the webcam/mic buttons like this one
https://www.svgrepo.com/svg/1558/computer-monitor-plain . Then toggle the webcam when screen sharing mode is entered/exited.

[hthetiot]

open-easyrtc/open-easyrtc#42 (comment)