* Add FLAC export, based on WAV renderer
* Depend on sndfile>=1.0.18 (previously >=1.0.11)
* Add compression option to FLAC export, if available.
The code related to compression is only generated if LMMS_HAVE_SF_COMPLEVEL is defined(libsndfile>=1.0.26).
* Save into the correct file extension upon single-export.
* Use unique_ptr in FLAC renderer and be more expressive about involved types.
* Use unique_ptr and remove manual memory management in ExportProjectDialog
* Add 'flac' format info to --help and manpage
