mscore font: add glyphs corresponding to SMUFL clef-number glyphs

The mscore font is lacking some of the glyphs in the SMUFL set. Clef+numbers glyphs are required to simply SMUFL integration.

Glyphs and code points added:

Treble clef 15 below: 0xe1d6
Treble clef 8 below:  0xe1d7
Treble clef 8 above:  0xe1d8
Treble clef 15 above: 0xe1d9
Bass clef 15 below:   0xe1da
Bass clef 8 below:    0xe1db
Bass clef 8 above:    0xe1dc
Bass clef 15 above:   0xe1dd

Metrics: same as for the base clef glyphs.

Numbers for the bass clef have positioned to clear the staff for standard 4th line position (transposing clefs are not really used for other position of the F clef).
Merge pull request #547 from mgavioli/mscore_font_add_SMUFL_clef_glyphs_2

mscore font: add glyphs corresponding to SMUFL clef-number glyphs