buildglyphimages: Make more versatile wrt directories

The script now expects a (relative of absolute) complete input file name.
This has to be in the definitions subdir of either the package dir or a copy of
the lilyglyphs_private structure.

The script now outputs the generated sources and the resulting pdfs in a single directory each,
not a hierarchy of folders corresponding to input file name.
This is because CTAN wants a hierarchy as flat as possible.

The generated latex file is put in a sibling folder (generated_cmd)
instead of a top-level 'stash' dir.
buildglyphimages: Handle existing command definitions

If the script processes a command and finds a corresponding
LilyPond source file it asks whether to overwrite that file
buildglyphimages: Update documentation

Document the changed script's behaviour
rebuild-pdfs: Adjust to flat directory structure

As done in the previous modification of buildglyphimages
the generated LilyPond sources and compiled pdf files are stored in
a single directory each.

rebuild-pdfs.py had to be modified accordingly
force new glyphimages/generated_cmd dir

... in main package and in lilyglyphs_private
remove obsolete dummy file from glyphimages/pdfs
buildglyphimages Move existing image command files to flat directories

The images and generated sources for image based commands
are now stored in a flat directory structure.

This commit moves the existing files to their new location
buildglyphimages: rename to genImageCommands.py

... to make it consistent with genGlyphImages.py
merge revision of image-commands script
Manual: Small touches on lilyglyphs_private
Manual: Update rebuild-pdf section

and document the expected CWD
rebuild-pdfs.py: Clean up
lilyglyphs-common.py: Clean up
Scripts: Clean up
manual: add linespread

With the given page margins and text font the default linespread was somewhat too tight.
Slightly extending it provides better legibility and line identification
manual: Set Numbers=OldStyle
manual: Enable broken tables in \reftable

The \reftable used to document predefined commands is now
a {longtable}. This allows tables to be broken around pages
and to be placed directly where they are inserted in the text.

Floating ins't acceptable in this case because there are so many tables
that having them float through the text makes it _very_ impractical
to use the reference.