Enable sharing an application window (instead of the whole screen) in video calls

[aral-matrix]

Is your suggestion related to a problem? Please describe.
Related:
https://github.com/vector-im/riot-web/issues/4880
#12652

Describe the solution you'd like
There should be a button next to the camera on/off button, that can be used during a video call to select an application window (or the screen) to be shared.

Describe alternatives you've considered
not really any alternatives I see

Additional context
n/a

[t3chguy]

Presumably this is for the electron app as this is supported in the browser?

[aral-matrix]

My bad! The suggestions template didn't ask for software versions. Yes, this is for the riot-desktop app (on debian), which appears to be what you refer to as the "electron" app.

[SimonBrandner]

I believe this can be closed since this already works in Chrome and Firefox and a fix for the desktop is waiting for review.

[SimonBrandner]

Oh, I just realized I hadn't read the full description. But making screen sharing more obvious is cover by #12652 anyway, so I guess this should still be closed

Edit: Sorry for the two comments my thoughts on this were confusing. My PR should close this, which seems to be the most sensible way to handle this