This fixes an -Woverloaded-virtual ambiguity introduced in b5e613d45
void render (cairo_t*, cairo_rectagle*)
void render (Cairo::RefPtr<Cairo::Context> const&, cairo_rectangle_t*)
ArdourCanvas prefers cairomm and CairoWidget itself uses Cairo::Context,
this improves overall API consistency.
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.
