Populate the dedicated view of direct messages ("People") in the rooms list

[kevinaboos]

Now that we have a new UI, this should be relatively straightforward. All we need to do is determine whether a room is a direct message, which can be done via multiple different SDK APIs:

• https://matrix-org.github.io/matrix-rust-sdk/matrix_sdk/ruma/api/client/sync/sync_events/v4/struct.SlidingSyncRoom.html#structfield.is_dm
• https://matrix-org.github.io/matrix-rust-sdk/matrix_sdk/room/struct.Room.html#method.is_direct

Then, those "direct" rooms should be placed under the People heading of the rooms list, and thus they should not appear under the Rooms heading any more.

Note that the collapsible section headers don't actually collapse/expand yet -- that is a known missing feature.