[WIP] added a basic beamline webcam viewer

Created by: meguiraun

It displays a button next to the chat, it is open on-demand by the user and displays the webcams defined in the config_gui (youtube video for mockups). Based on iframe, it works for us but let me know if this is not working for your cams, or it gives problems.

I know there is more work to do in this topic (like open automatically when sample loading, switch canvas, etc.), this is a first approach to have a webcam in mxcube3.