Add directory for LilyJAZZ
lilypond-version-switch.ly: Add predicates

Update description and add remaining predicates.

TODO:
- rename to lilypond-version-predicates.ily
- move usage examples to new .ly file
- is calculate-version correct? Can't imagine that.
I'm sure I had something different. Have to scale the three parts.
Fix lilypond-version-switch: wrong calculation

Somehow I had forgotten to include the right calculation
int the original commit :-(
Rename and move lilypond-version-switch, make includable

Add new directory 'general-tools' and moved the snippet there.
Rename to lilypond-version-predicates.
Make snippet includable by moving the usage example to separate file.
color-voices: Remove obsolete \revert-s
color-voices: Rename to .ily and add Flag color
color-voices: Add usage example file
make aligning-first-lyric-syllables includable
make adjust-horizontal-spacing includable
Make align-lyrics-on-vowels includable
Make generalized-offsets includable
Make manual-partcombine-for-vocal-parts includable
Make metric-mod-function includable
Turn scaling-stencil into a music function
Make scale-stencils includable
debugging columns: place debugging info on top
Update README.md
Merge 'shape-update' into master.

This introduces two new, powerful versions of slur shaping
function originally written by David Nalesnik.  They may later
be expanded and possibly merged into one ultimate slur-shaping
function.
move snippets to their own subdirectories
remove "custom-engravers" category

I've decided that the snippets there fit the "notation-snippets"
category.  Being an engraver or a function, or something else
altogether, shouldn't be a basis for categories.