Secure and port custom tags when changing settings from 'mb library' to 'file'

[hiccup]

A suggestion for improvement since issues concerning this have come up more than once lately:

When for a custom tag you change it's 'save to' properties from 'save to musicbee library only' to 'save to music file as a tag' (or the other way around), the already present tag values are not ported and will get lost.
That's neither desirable nor expected.