Keep tiles in a stable order

[robintown]

This introduces a new layer of abstraction on top of MediaViewModel: TileViewModel, which gives us a place to store data relating to tiles rather than their media, and also generally makes it easier to reason about tiles as they move about the call layout. I have created a class called TileStore to keep track of these tiles.

This allows us to swap out the media shown on a tile as the spotlight speaker changes, and avoid moving tiles around unless they really need to jump between the visible/invisible regions of the layout.

Closes https://github.com/element-hq/element-call/issues/2562