Threads: separate thread timeline view (in dock tabs), thread summaries, threaded replies

[kevinaboos]

• Introduce the concept of multiple timelines per room:

  • 1 main timeline, and only one
  • N threaded timelines, one per thread root event (the message from which a thread starts / branches off)

• Enable threaded sessions, clients, and all other thread-related SDK configurations.

Remaining fixes/todos before merging:

• use the new TImelineKind enum everywhere
• set_avatar_and_get_username() uses the new TimelineKind enum
• All other MatrixRequests (that need it) use the new TimelineKind enum
• replace the usage of room_id (OwnedRoomId) as a timeline selector/identifier with the proper TimelineKind
  • this includes the room_id in the TimelineUiState itself as well
  • this also includes anywhere that we see both a room_id and a thread_root_event_id together. Those should be combined into the TimelineKind::Thread
• re-format the thread root summary view DSL to be proper (not a bright red marker, lol)
• Manually query thread root summary for an updated reply count and latest thread reply, because SDK-provided functions do not work consistently.

Future to-do items:

• Now that we have per-thread timelines, ensure that any action that was previously gated by checking for a room ID match now either checks the TimelineKind, or just captures the actions properly. Only handle ShowUserProfileAction in a single room screen #757
  • For example, the ShowUserProfileAction only checks room ID.
• Fix the unread badge on the jump_to_bottom_button in a thread-focused timeline. (Don't show the unread badge on the jump-to-bottom button for threads #759)
  • I don't think there is a way to obtain the unread message/mention count for a specific thread, the matrix SDK only offers it for a main room timeline.
  • Thus, we should probably just hide the unread badge on the jump_to_bottom_button entirely for a threaded timeline.