Revert "Remove the lock, collapse and hide option for dock titlebars"

We removed the docker titlebar widget in b662043a because it gave problems on macOS. Since then, the animation timeline got a custom titlebar, and it seems that doesn't present any problems anymore, so we could try to restore this functionality.

Note: it's still debatable whether we should restore the "collapse" functionality as well, because that confused a lot of people.

Ivan, can you please test on macOS?