window: Support animated and touch scrolling

Add back code to conditionally compile TerminalScreenContainer
to use a GtkScrolledWindow instead of creating our own GtkScrollbar.
When used, this gives us animated and touch scrolling for free.
We can mimic the existing behaviour by creating a GtkScrolledWindow
that never scrolls horizontally, but always vertically. We can
use the new EXTERNAL policy to hide the vertical scrollbar widget
depending on the setting.

However, GtkScrolledWindow doesn't work with
GTK_SIZE_REQUEST_CONSTANT_SIZE children, so this code will only
exist for development purposes until gtk+ fixes this.

Also install custom CSS to hide GtkScrolledWindow over/underscroll
animations.

Also reverts commit 41eb2ec7

https://bugzilla.gnome.org/show_bug.cgi?id=733210