different approach to independent scrolling, involving ArdourCanvas::ScrollGroup

The idea now is that a scroll group item can be added to the canvas which will causes its children to scroll in either or both
directions (horizontal or vertical). There are few complications: the position() of the ScrollGroup is ambiguous depending
on whether you want it with scroll taken into account or not, so Item::canvas_position() was added, which defaults to
the same value as Item::position() but is overridden by ScrollGroup to return the position independent of scrolling. This
method is used when translating between item/canvas/window coordinate systems.

Note that the basic idea is that we MOVE the scroll group when a scroll happens. This mirrors what happens in the GnomeCanvas,
where Nick Mainsbridge came up with a great idea that allowed unification of the time bar and track canvases.

libs/canvas/wscript

@@ -50,6 +50,7 @@ canvas_sources = [
         'polygon.cc',
         'rectangle.cc',
         'root_group.cc',
+        'scroll_group.cc',
         'stateful_image.cc',
         'text.cc',
         'types.cc',