GtkRange: Add a style class during dragging

This will be used to identify a scrollbar is being dragged - we
don't actually need the style class; another way to keep track
of the dragging status would be ok too.
Add an experiment for overlay scrollbars

This is just meant as a prototype for how we can show a
scroll position indicator for touch. This uses GTK_POLICY_EXTERNAL
to hide the scrolled windows scrollbar while still allowing
scrolling.
GtkScrolledWindow: Add overlay scrollbars

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.
Add some overlay scrollbar theming to Adwaita
Port the overlayscroll example

Make the overlayscroll example use the overlay scrolling that is
now built into GtkScrolledWindow, instead of the manual prototype.
GtkScrolledWindow: Add documentation

Document the new overlay scrolling APIs, and add some more
documentation for the other touch support: kinetic scrolling
and overshoot.