…te fields of `RoomInfo`

…pt state

Before this patch, we needed to clone the inner `timeline_queue` and turn it into a concrete `Vec<SyncTimelineEvent>`, just to iterate on the elements,
and because returning an iterator from a trait method is impractical. This now changes it to return the actual concrete type of `timeline_queue`, so
we don't need the extra allocations.

Ideally, matrix-sdk and matrix-sdk-base would be merged, so we don't need to use a trait at all here.

This helps supporting cases where we want to show that a room has some activity (unread messages) but no notifications.

- copyright notice
- doc comments and better doc in general
- use static dispatch instead of &dyn T
- other misc comments

The test fails only in the codecov build, not in a local build or in the other integration test.

Needs further investigation.