qml: control bar delayed calculation should be done at appropriate time

The calculation is not necessary, when when is not set. So, there is no need to do the calculation.

At the same time, the calculation must be done after when is set. Currently, there is no guarantee for that and it causes issues with expanding items getting invisible unexpectedly.

Request review @chub.

As a side note, there have been some attempts from Qt's side to change the type of value to QQmlScriptString, but it has never been merged.