qml/ControlLayout: Update resize and collpasing behavior

This MR improves collapsing for the ControlLayout loaders based on their internal content. This results in a more optimal space distribution among control items. It's notably useful to gain space when a loader is empty like in the classic controls scenario.

NOTE: We probably have to update the backward / forward and volume slider expanding behavior.

NOTE: We should update the expanding spacer behavior or remove it entirely.

depends !3464 (merged)