scrolledwindow: make it a windowed widget again

This is so we can detect crossing events on the scrolledwindow widget,
which can be useful for toggling certain behaviors or not depending on
whether the mouse is on the widget.
scrolledwindow: fade out scrollbars on leave notify
scrolledwindow: improve v/hscrollbar proximity detection

Only horizontal coordinates are checked for the vertical scrollbar, and
vice versa.
scrolledwindow: Enable "hover" mode in scrollbars after a timeout

This way transient mouse movements over scrollbars don't cause flickering,
and more spaced movements have it enabled shortly.