Export and import tags for all songs in library

[madmaxoft]

In Library Maintenance, it should be possible to export all songs' tag values to a file. This will export the best value for each property of the tag (PrimaryAuthor, PrimaryTitle etc.) for all songs.

As a matching action, an import should be made that would take this pool of tags and assign to any missing property in the current library. It should NOT overwrite existing tag properties. It should write to the Manual fields, so that it's fast and doesn't change actual files. It should be left for #158 to copy from Manual to ID3.