Assign selection colors to users. goals:
    * current user is always main color
    * every other user has the same color in every context
    * users don't need to be in a shared project to have a color. they can either be in the call, or in a channel notes.
    
Places colors are used:
    * editor element, driven by the buffer's `remote_selections`
    * pane border (access to more state)
    * collab titlebar (access to more state)

Currently, editor holds an optional "replica id map".

Most challenging part is in the editor, because the editor should be fairly self-contained, not depend on e.g. the user store.