More robust Tags plugin MPD filter configuration

More robust Tags plugin MPD filter configuration

Remove external enclosing parentheses in user defined MPD filter, for
instance when there is more than one expression:
    ((genre == 'rock' ) AND (date =~ '198.')).

Even though it's a valid MPD filter, forge_filter will enclose it
properly. We do not want to through a syntax error at users since it's a
valid MPD filter, hence trying to transparently reformat the filter.

filter = ((genre == 'rock' ) AND (date =~ '198.'))

  and

filter = (genre == 'rock' ) AND (date =~ '198.')

Will work the same, producing the valid MPD filter:

    ((genre == 'Punk') AND (date =~ '198[2-9]+') AND (artist != ''))