PageRow: Do not async load the global header
The calculation of the PageRows' implicitHeight does not take the header title into accont until it is loaded.
If the PageRow is used inside a QQuickWidget with resize policy SizeRootObjectToView (as in KCModuleQML) the initial sizeHint() will use an implicitHeight that is not ready yet.
BUG: 442660 FIXED-IN: 5.86