DefaultListItemBackground: Respect app-overridden controlRoot paddings
Right now ItemDelegate and DefaultListItemBackground include some built-in padding. This makes sense for views that have 0 internal padding. However sometimes you want to set the padding to 0 because the view has its own internal padding, and you don't want the list items to get double-padded.
Currently if you do this, the layout gets broken because DefaultListItemBackground does not take its parent item's paddings into account properly.
This commit fixes that. There are no visual changes for apps using the default paddings.