Hopefully finally fix unreads

[VAWVAW]

Just use sdk functions to track unreads.

And send read receipts according to spec:

• no receipts for our own events
• no receipts for state events
• private read receipts if we don't want public receipts

This also adds the distinction between

• highlights (the user in mentioned)
• notifications (what should be shown as unread)
• messages (in muted rooms)
  and doesn't highlight the last one in bold in the room list.

fixes #564
fixes #553

Edit: Also add a :mentions window.
fixes #382