qt: regression: flaky organization behavior in control bar

This is hard to explain so I have recorded a video:

vlc_0lSk8OR4z3

1. Start the application.
2. Make control bar visible.
3. Start the toolbar editor, do nothing, close.
4. A control appears.

Expected behavior: The newly appeared control should have been visible when the application first started. This is because there is enough room that can be gained from the expanding control(s).

Note that this behavior can be observed in other circumstances as well. I believe it is due to no longer using a declarative approach. This can get quite messy to handle without going a declarative way.

Regression since !3385 (merged).