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.
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