Problem: ext_messages cannot tell when the screen was cleared, which is
needed to clear visible messages. An empty message is also
never emitted, but clears messages from the message grid.
Solution: Repurpose the "msg_clear" event to be emitted when the screen
was cleared. Emit an empty message with the `empty` kind to
hint to a UI to clear the cmdline area.
2b4c1127ad
