Fixes bug #6179. Top vs. bottom seems pretty arbitrary to me, and this solves
the obscuring issue (which is quite common since there are often PC events at
the start of MIDI files), so bottom it is.
Remove Canvas::Layout, use Canvas::Container for the same purpose, move child-rendering into Item::render_children() so that it
could theoretically be used by any derived type.
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.
