Issue 3186: fix clef transposition alignment

Issue 3186: fix clef transposition alignment

Until now, numbers indicating clef transposition (for example
in \clef "G_8") were simply centered against the clef glyph.
This was not optimal, because clef glyphs aren't symmetrical
- for example the hook of the treble clef isn't exactly at its
horizontal center, and thus the octavation number shouldn't
be exactly centered.

This commit allows to specify alignment separately for each
clef, taking into account their individual characteristics.
Alignments are stored in clef-alignments property.