Add support for more than one voides.

Current implementation has only statically applied treble clef.

Add a sub-field or text formatting to allow support of different types of clef in a single Stave.

Remaining tasks

1. Few ideas:
   1. Javascript editor like wysiwyg
   2. Use of separators etc.
2. Modify the field widget according to step 1.
3. Modify the notation rendering javascript(vexflow-render.js).