ma-chat: Emit changed when unread count changes

Otherwise the unread counter shows outdated data. Flow so far was as follows:

• User selects a room
• Chatty invokes set_unread_count
• cm_room_set_read_marker_async updates the read marker server side async
• [nothing updates the chat information]

On second click the current unread count is read and the marker updated.

Now we update the count as soon as we get informed from the server that the unread count got updated.

I opted to ignore other glitches like the currently active room getting a non zero unread count.

Needs https://source.puri.sm/Librem5/libcmatrix/-/merge_requests/72 to work.

Closes: #908 (closed)