Draft: Allow setting the view background color for ScrollablePage's main view

This facilitates various pieces of software that use a Kirigami.ScrollablePage for a scrollable view where the lighter view background makes sense. For example, when the content view is a scrollable list of some sort, or a large view of text. These apps currently override the background with view-colored Rectangles, which is a bit messy. It's nicer to support this use case in the component itself.