Audio-only share option

implementors: please make this just a special case of a video share instead of an Entirely Distinct Thing Internally. i also share "just audio" sometimes. when i'm screensharing, i only use niri's "dynamic cast" functionality, which lets me easily switch which window I'm sharing using compositor hotkeys. but i can also set it to share nothing at all; at which point no more frames are sent to the recipient. different apps handle this differently, from freezing the last shown frame, to just displaying black, to "buffering" (even though there are no network issues), to just forcefully shutting off the stream because it assumes something broke. it would be really nice if Fluxer could instead just turn my screenshare tile into one that says "audio-only", and you could still interact with it (change volume / stop sharing / etc) just the same as a video screen share, and then when i select a window to share again it will just continue without interruptions. switching between audio/video share on the fly should be seamless for viewers/listeners. i should not have to restart my stream entirely to have "the other mode".