This commit adds a mode to GtkScrolledWindow in which it puts
narrow, auto-hiding scrollbars over the content, instead of
allocating room for the scrollbars outside of the content. 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. The
environment variable GTK_OVERLAY_SCROLLING can also be used
to force overlay scrolling on or off.