Improve consistency between QWidgets and QML KCMs margins

Previous layouting relied on the page GridLayout's spacing and global contents margins to set components spatially. This breaks for Qml-based KCMs, as the Qml component sets its own internal title, spacing and margins.

Now, we remove contents margins for Qml-based KCMs, while keep them on QtWidget ones, to keep them visually unchanged.

This will hopefully help with consistency between different standalone KCMs, systemsettings sidebar view and systemsettings icon view.

Co-Authored by: @ngraham (!36 (closed)).

It depends on kwidgetsaddons!56 (merged), to be able to access the KTitleWidget item from the module code.

QML	QtWidgets