Softcode alteration/accidental symbol link.

Softcode alteration/accidental symbol link.

This allows different microtonal conventions by overriding the
'glyph-name-alist property of accidental.

Related changes:

- Use rational for grob property 'alteration

- Soft code alteration_pos function for  key signatures.

- Do away with systematized names for accidentals in MF font. Provide
descriptive names instead. Convert-ly rule.

- Add (mirrorflat+flat) and sharps with three beams for Makam notation

- Remove 'style property of accidental. Rather, override
  glyph-name-alist instead.