Improve multiple screen share indicators usability

### Your use case

#### What would you like to do?

Improve the current usability and user experience of the multi-screen share experience.

#### Why would you like to do it?

Element Call allows more than one user to screen share during a call. This is a **great** feature!

Unfortunately, the visual indicators for this are not intuitive to most users.

Indicators are shown as 2px height white lines below the first screen share, on the lower left corner, and a chevron is shown to the right of the screen share, which only becomes apparent if you hover it.

For most new users, this is a clearly not intuitive and most only understand how it works once it is explained verbally to them.

#### How would you like to achieve it?

Screen real estate is important but usability is more. I propose to replace the current indicators with a call to action button with a common screen icon and the user's display name concatenated with "'a screen share", such as this:

Clicking the buttons effectively changes the active screen share. The chevron button remains as an alternative.

### Have you considered any alternatives?

_No response_

### Additional context

I will be submitting a PR that implements this as a starting point to moving forward..