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 -> = {}.
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
(bool) false == 0 == (const char*) NULL
error: ISO C++ says that these are ambiguous, even though the worst
conversion for the first is better than the worst conversion for the second:
actions.h:92: note: candidate 1: Glib::RefPtr<Gtk::Action> ActionManager::get_action(const char*, const char*, bool)
actions.h:91: note: candidate 2: Glib::RefPtr<Gtk::Action> ActionManager::get_action(const std::string&, bool)
