Add a scrollable indication

Draw a themable indication when an edge of a scrolled window
is hiding some content that can be scrolled in.
Adwaita: Initial theming for scrollable indication

This just uses a narrow shadow for now.
Add a gtk_scrollable_get_border

Add a vfunc to return a non-scrollable border around scrollables.
This would be nicer as a property, but we can't add properties
to an interface without breaking 3rd party implementations, so
make this an optional vfunc, and handle it not being set.
GtkTreeView: Implement get_border

Add an implementation for the new scrollable vfunc that returns
the header height as top border.
GtkScrolledWindow: Take border into account

Use the new scrollable API for getting non-scrollable borders
and draw over/undershoot at the right place. In practice, this
means that they now appear below treeview headers.