Option to switch between audio, video and screen sharing

[ghost]

Is your suggestion related to a problem? Please describe.

There seems to be no way to switch to video call, or screen sharing when you already in a audio call.
Also it is not possible to stop/pause screen sharing without stopping the complete call, the camera toggle has no effect while screen sharing.

Describe the solution you'd like.

Only one button for starting a call. After starting a call - a confirmation prompt with options for microphone mute, video or screen sharing or audio only. This confirmation prompt should also appear when accepting a call -> #17152.
The options for the video source should also be available during any call, to seamlessly switch between audio only, video or screen sharing.

Describe alternatives you've considered.

At least the camera toggle button should be able to deactivate the screen sharing.

[SimonBrandner]

Duplicate of #12646 and #12652. Where the latter is being worked on at matrix-org/matrix-react-sdk#5992