Add mass copying of Manual tag into ID3 tag

[madmaxoft]

In DlgSongs there should be a way to copy manual tag into ID3 tag for selected songs.

[madmaxoft]

If the source is invalid or empty, the target shouldn't get overwritten

[madmaxoft]

Perhaps add a specialized tool for copying tags on all songs in the library in all reasonable ways:

• Manual to ID3 (possibly slow)
• ID3 to Manual
• FileName to Manual
• FileName to ID3

The tool would need some decision-making about overwriting existing values, and possibly limiting the scope (invoking the tool on DlgSongs's selection?)

[madmaxoft]

Copying to Manual has no real use case (after #198 was implemented), so the only remaining valid operations to be implemented are:

• Manual -> ID3
• Filename -> ID3.