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 fixes mp3 export on Linux/ARM (Raspberry Pi) with
system-wide dynamically linked ffmpeg. Otherwise the there
would be library conflicts with ardour-bundled libz and libcairo.
This uses the existing optional runtime ffmpeg dependency to add support
for m4a files (and in theory whatever other file formats ffmpeg
supports) to the import audio dialog.
The same functionality is mostly already available via Session -> Open
Video, with the "import audio only" selection (even though m4a isn't
currently included as one of the video formats, it still works). Having
this in the import audio dialog however seems much more user friendly.
