Introduce a helper function

Instead of making assumptions about enum value ordering,
introduce a helper function to determine policy values
for which the scrollbar may be visible.
Add a since
Add an initial implementation of overlay scrollbars

This commit adds a mode to GtkScrolledWindow in which it puts
translucent scrollbars over the content, instead of allocating
room for the scrollbars. We use traditional scrollbars if we
find a mouse or if overlay scrolling has explicitly turned off.
For test purposes, GTK_TEST_TOUCHSCREEN can be used to get
overlay scrolling even in the presence of a mouse.
Add some overlay scrollbar theming to Adwaita