[Feature request] Support live video sources

[mstarongithub]

Supporting displaying the output of live video sources, such as webcams or video capture cards, would be useful for an easy way to, for example, double check that your webcam works or to play some game on a console, streaming it to your PC.

In MPV, you can achieve the result via mpv /dev/video0 as example

[Quackdoc]

This works, you just need to use a full uri cosmic-player v4l2:///dev/video0