Added structural validation framework and some rules.
Added vocab validation, also serves as documentation.
Added placeholder for clef.

Changed vocab:
 - accidentals includes :no-accdidenatl and :natural
 - added single major mode for now
Added clef processing and clef-aware note placement.
Changed the coordinate offset for stave render to middle line in layer 3-render.
Added clef-offset to layer 2-3.
Changed representation of y coordinate in layer 3 to the centre line.
Placeholder glyph for clef.
Added correct placeholder for key signature.
Changed demo to use both clefs, plus screenshot.