Rework MessageDelegate to be mostly a cpp class. This allows us to only load the components that are actually needed saving memory.
In testing using memtest it saved ~30% versus the current implementation.
Create a list model for read markers. The primary reason is to stop `RoomMembers` being accessed after their state event is deleted. With this the read marker doesn't pass and `RoomMember` objects to qml.
Depends on https://github.com/quotient-im/libQuotient/pull/695
Currently basic just to show a working implementation using RoomMember. Currently only the room event and search models are moved over. Will change everything else over once the dependent pr is complete.
