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