layout: Make sure startup animation completes

We currently complete the animation using an onComplete handler, which only runs if the corresponding transition was stopped when finished.

While it is unexpected that the transition is interrupted, it can apparently happen under some circumstances (like VMs with qlx). The consequences of that are pretty bad, mainly due to the cover pane that prevents input during the animation not getting removed.

Address this by always completing the animation when the transition is stopped, regardless of whether it completed or not.

Fixes #5337 (closed) (presumably, needs actual testing to confirm)