qml: do not use paddings in stack view

Qt Quick Stack View, even though is a Control derivative, does not respect its paddings. This does not mean that we are free to re-interpret the paddings as done currently. Qt may at any point start respecting the padding, which would in turn break the interface.

The padding is not supposed to be passed around like that. It is for the control to position the content item (apply margins). Unfortunately we can not do that in StackView because it assumes that it is the sole authority for positioning the content.

The easy fix for that is to simply use anchors where StackView is reused. This is what I tried to do in this merge request.

Unfortunately I could not override the padding properties as read-only to block adjusting them, as they are marked FINAL.

This was first discussed in !5890 (comment 479037).

Request review @chub.