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 -> = {}.
pbd/i18n.h MUST NEVER be included from header files and always be
the last include. This is because `_` is declared other headers
notably boost and some apple headers.
leading to issues like
../libs/pbd/gettext.h:58:27: error: expected unqualified-id before ‘const’
58 | # define gettext(Msgid) ((const char *) (Msgid))