db-add movse files to a staging area and updates index

This approach would allow multiple local modifications to be aggregated on disk before a commit to the database is performed. Nevertheless, it might actually be desirable to aggregate changes into a local commit directly.
db-add: cleaned up code

- Added common/Date.hpp class that streamlines the date acquisition task.
- Added db/Tracks.hpp that reads, holds and writes the track list structure to/from a stream.
- Cleaned up db-add.cpp to not contain code regarding the index anymore.