Raw tag management analogous to foobar2000

[mallniya]

As discussed in #115, one should have access to any tag field in tag editor, which I suppose making $meta(tag) behavior analogous to $meta() in foobar:

$meta(name) Returns value of tag called name. If multiple values of that tag exist, they are concatenated with ", " as separator. Example: $meta(artist) → "He, She, They"

$meta(name,n) Returns value of n-th (0,1,2 and so on) tag called name. Example: $meta(artist,1) → "She"

I guess this also implies showing all tags of a track in Selection info panel.