incorporate drawable and annotations

this removes most of the contents of GlSequence in favor of drawable,
and adds shared_ptrs to both drawable and annotations into SeqSpan.

One problem is getting the code to do coping instead of sharing a
reference at the right time.

What I've got right now is I added some constructors that take ThingRefs
(aka boost::shared_ptr<thing>) and those will create copies, while
the more standard reference or pointer initializers just copy the
shared_ptr.

This was a bit problematic in the case of GlSeqBrowser, which really
needs to make sure that its glsequences have their own drawable component.