the rest from `tools/convert_boost.sh`.
* replace boost::function, boost::bind with std::function and std::bind.
This required some manual fixes, notably std::placeholders,
some static_casts<>, and boost::function::clear -> = {}.
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.
A Gtk::manage()d widget will be deleted when its parent container
is destroyed. Top-level context menus are not inside a container and
hence need to be manually deallocated.
The solution here is to use a shared Gtk::Menu pointer that is
centrally de/re-allocated.
This works because the GUI is single-threaded and at most one
context menu is visible at a time.
