Explicitly layout the window content item vertically as well

This worked implicitly so far as Qt's ApplicationWindow sets the bottom padding to the footer height plus the safe area bottom margin, the latter being the safe area fallback handling introduced together with the safe area API.

This breaks for applications wanting to opt-out of safe area fallback paddings and actually draw content in unsafe areas as well, as that's done by explicitly setting the window padding to 0. Doing that will allow to draw inside the unsafe areas, but also expands the window content item below the footer.