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.
This makes continued merges with ardour codebase easier, at the
small cost of removing the XML-based constructors. They were not
in use anyway and their use doesn't really fit into the
conception of the canvas as a dynamic UI element. Could reintroduce
them at a later date.
note: the solution is non-obvious, but the obvious one of applying a translational Cairo::Matrix to the gradient
caused an exception to be thrown because of a non-invertible matrix.
