Copyright-holder and year information is extracted from git log.
git history begins in 2005. So (C) from 1998..2005 is lost. Also some
(C) assignment of commits where the committer didn't use --author.
- don't add a new SysEx canvas item every time we zoom or drag.
- speed up redisplay generally using PatchChange-like method
for finding items (find_canvas_sys_ex() in boost::unordered_map).
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.
