theme: Add a top border to statusbar

From the main commit message:

Since a statusbar is usually placed along the bottom of a window, it would be better to have a top border by default. (Most other container "bar" widgets, such as actionbar, headerbar, menubar, infobar and searchbar, already have their own side border by the theme.)

This eliminates the need for application developers to manually add a separator/frame/CSS border to separate it from the main content.

This merge request also removes a separator and frame from demos to avoid double borders.

In widget-factory, this properly hides the statusbar separator when the statusbar is hidden via the "Menubar -> View -> Statusbar" action.