5-string banjo does not implement the truncated ("drone") 5th string

[kamalmostafa]

The 5-string banjo's fifth string starts at the fifth fret, but Fretboarder doesn't account for that. In Fretboarder, the indicated notes on the fifth string are all incorrectly shifted by five frets.

Implementation idea: Keep a "zero_fret" member for each string, set to 0 for all guitar/banjo/violin/uke strings except the banjo's fifth where its set to 5. Apply the zero_fret offset to all note/interval calculations. Draw the banjo's fifth string only up to the fifth fret.