Adjust grace note slash and dot handling

[dbenjaminmiller]

I've fixed two issues, although the code might be made a bit more elegant.

1. Ekmelos has an unusually small grace note slash compared to other SMUFL fonts. See the comparison at Grace note slash the wrong size? ekmelos#4. I've changed the font size of the grace note slash so that it looks proper with other smufl fonts; the glyph in Ekmelos should probably be adjusted accordingly.
2. Augmentation dots placed too far to the right #7 - I've fixed this issue by changing how we draw augmentation dots. The existing implementation works by replacing the entire Dots stencil (which draws one augmentation dot where there is one, two where there are two, etc.). The spacing of these dots, however, didn't match that which LilyPond gave natively; most notably, the first dot was further away from the notehead than LilyPond would put it. But instead of handling the dot placement, I switched it so that we no longer override Dots.stencil but instead Dots.dot-stencil (the stencil for a single dot). This allows LilyPond to handle the spacing while still using the SMUFL font's dot character.

[tr-igem]

Yes the size 3 for grace note slashes is wrong. I removed the fontsize command completely (it is unnecessary for 0) and changed also the positioning on the stem according to bravura_metadata.json.

[tr-igem]

dot-stencil simplifies the dot handling (seems to be a new property in Lilypond 2.24).