VoIP/Call UX/UI wishlist

### Introduction
Currently, there is a great amount of work being done on VoIP :+1: This issue aims to, summarize the missing UX/UI bits, which I think are required to reach the *perfect* user experience. I will update this as new issues are created, and I get new ideas.

This has some overlaps with [#3025](https://github.com/vector-im/element-meta/issues/1487). The focus of that issue is overall functionality whereas the focus of this issue is the way the functionality is allowed by the UX/UI.

**Note:** Some of the things mentioned here aren't useful at this point. They would become important if native Matrix groups calls were implemented. I am not sure if that is ever going to happen.

### Wishlist

+ [ ] Different view types, focusing on feeds...
+ [ ] Focus on a specific feed
+ [ ] Grid (Zoom-style)
+ [ ] Main feed and a scrollable bar with secondary feeds (Zoom-style)
+ [ ] Main and secondary feed; the position of the secondary feed should be customizable; examples (main + secondary): remote webcam + local webcam. remote screen sharing + remote webcam
+ [x] Implement screen sharing properly (not the current "easter egg" way)
+ [ ] Ability to hide the local feed
+ [ ] Ability to automatically set the main feed to be the person who is speaking
+ [ ] Ability to change speaker, mic and webcam during a call
+ [x] Show call length/duration/elapsed time (Android [PR](https://github.com/vector-im/element-android/pull/2575))
+ [ ] Control volume of call participants
+ [x] Indicate that a person is speaking
+ [ ] The only difference between the voice/video call buttons should be the state (on/off) of the webcam when the call starts) - the user should be able to turn on their webcam mid-call