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: Use get_border

Instead of poking directly at treeview internals, use the new
scrollable API for getting non-scrollable borders.