Items no longer need a parent group (they require a Canvas pointer instead), so all constructors have been rationalized
and have two variants, one with a parent and one with a canvas.
All Items now inherit from Fill and Outline, to banish diagonal inheritance and virtual base classes and all that.
There were zero changes to the Ardour GUI arising from these changes.
Delivery of fake motion events to the editor needed the event coordinates to be
in canvas space, as they are with "real" events. Editor and other objects had
many redundant groups from timbyr's work on gnomecanvas to scroll by moving
groups. We don't need this anymore with cairo-canvas (though possibly a
stationay background group for the canvas might be useful again one day as in
the SAE logo. Its implementation would be fairly different though, since we
would have to explicitly move the group on every scroll, since nothing else
ever moves on scroll).
Also tweaks to text item placement, and switch TimeAxisViewItem from
name_pixbuf to name_text, since ArdourCanvas::Text is already "pixbuf optimized".
