timeline: fix incorrect headers on encrypted messages

The logic used to display headers in the timeline relies on the Event::can_hide_header method. This method returns false for encrypted messages. Decryption is performed asynchronously so it is possible for the view to be rendered before the decryption has completed, therefore can_hide_header returns false for messages which should in fact have a header.

This commit adds a "decrypted" event which is emitted by Event. Timeline listens to this event and updates the header states of messages on receiving it.