Adwaita: Style separator.sidebar

Adwaita: Style separator.sidebar

Let separators be declared as sidebars to have the same style as those
drawn by GtkStackSidebar. This also let them handle the selection-mode
class, whether they are assigned it or they descend from something in
selection mode.

Also drop setting the selection mode color for non-sidebar separators.

This is convenient when building a custom sidebar using a GtkSeparator
and to extend a sidebar to the title bar.