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 reverts commit dfdb91b429
and commit 188d766757.
despite earlier assumptions the code is correct and there is
likely a memory corruption elsewhere.
