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 -> = {}.
* It was never called because SourceListBase::set_session
subscribed to the signal just before SessionHandle::set_session
dropped _session_connections
* remove_source() only checked the first source of the whole-file
region, not all sources of a multi-channel region.
Stereo regions with mono sources are now properly cleaned up.
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.
