BusyIndicator is never shown on Kirigami PairPage

[jmlich]

It seems that BusyIndicatorPL isn't visible even when I override the property like this running: true. I have found that this is related to Kirigami.ScrollablePage behaviour and I was able to make it visible using background property of the ScrollablePage

+ background: BusyIndicatorPL {
- BusyIndicatorPL {

However, I am not sure how to make it generic for other flavors.

I have attached the pull request to make clear what changes are needed.