qml: do not delay setting topInset of ControlBar

The topInset is used for a specific purpose here, it does not matter if it is included in the implicit height calculation.

It would be nice to make it flexible such that if background is replaced by another component with implicit height, then take topInset into account, but then we can expect the implicitHeight to be changed as well so it should not be a problem.

This will correct the momentary height change of the background observed when getting into player view.