Get rid of custom incubator, make initialProperties map truly initial
That custom incubator does nothing other than binding initial properties too late compared to what real initial properties should be, so `required` properties were not possible in KCM sub-pages. More over, QML Engine/Component are using QVariantMap while the custom incubator used QVariantHash for some reason. I've been told that it was need as a workaround for some Kirigami issue, which isn't relevant today anymore. Luckily, no public API is affected by this clean-up.