Export MusicScanner
[tools] Add musicscan utility
Do not leak Tag instances during metadata scan
Remove unused link_libraries
Make showing scan progress optional
Initialise m_cmdQueue
Be able to do a dry-run scan
Report every scanned file.

Although this will make the logfile larger it will give us much better
hints which file we are failing on in th e case of crashes.
[tools] Add support for scanning a whole directory
Initialise MusicScanner::m_scanned