This draws heavily on what @carlschwan did in network/neochat!1579 but I found it easier to start again and grab the bits as I needed them plus some other copying from what I did in the Space tree model.
From my current limited testing this seems to work nicely try and break it.
Move the various room models into RoomManager. This means the same room models are always used and is a base from which further logic can be moved from QML to cpp.
Make sure that the new rules for counting notifications for muted, mention and low priority rooms is applied consistently to the room list, space drawer and the task manager notification badge
implements #644
Change the LastActivity sort order to Activity and update to a more flexible way of sorting based on an order from model roles.
Add options to actually switch between Alphabetical and Activity
Based on some old work by @tdfischer
implements #103
