[WIP] Add WebRTC features to Linux

[Shotman]

This PR is a starting point to enable media/WebRTC features to Linux currently it allows to get to this point :

But the issue lies with the navigator.getUserMedia() this API is deprecated and not available in webkitGTK and now navigator.mediaDevices.getUserMedia() must be used, and using some polyfill or dirty JS injection doesn't solve the problem. I'm putting this as a draft PR if one day either Discord or webkit changes something so it can be merged. In the mean time if someone has an idea you are welcome to modify the code.

[SpikeHD]

FYI for anyone coming across this, if I remember correctly from testing, the core problem lies in the fact that RTCPeerConnection is undefined on instances of webkitgtk without the WebRTC flag.