Regular .h files *should* be self-contained and independent of previous
includes and guarded to only include once. Make it clear which files
that *doesn't* apply for at all.
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 necessary with clip recording because for some short time after recording,
a trigger may be playable despite not yet having a region.
libs edition.
