Work on #49: Find stale pdf images

Generalize rebuild-pdfs.py

NEEDS ATTENTION: Doesn't seem to work.
Please check where directory names are used and where not when 
collecting file names
Add empty test file to have dupes in the directory
Continue Python scripts: Add options

This is still quite a mess. Just committing/pushing because of change of computer
Remove dummy file crescHairpin.ly again
Maske rebuild-pdfs work again after generalization

TBD: Clean up source files
(I don't want to do this in the same commit, as I'm not sure what this could break.
So I want at least the result of this commit in working order)
rebuild-pdfs.py: Clean up lilyglyphs_common.py

To be done: Check for side effects for other scripts
Work on generalizing buildglyphimages.py

ATTENTION: Still stuck
Please redo the work-flow with the new 'commands' dict.
If that works, remove the old references
Add tied whole notes
Scripts: create Eric4 project
Rename glyphimages/pdfs to generated_img

in order to allow for a later inclusion of manual_src and _img
Rename py/buildglyphimages.py to build-image-commands.py
Scripts: Start rewriting with Classes

It seems that my dictionaries become too large and difficult to handle
so I restarted with classes.

BaseClass Commands is subclassed by ImageCommands.

build-image-commands.py seems to already work,
but the other scripts have to be changed accordingly :-(
Scripts: Re-implement buld-image-commands with OOP

Finish design of classes.
build-image-commands.py seems to work again.

Next step: remove unused material from lilyglyphs_common.py
-> this will surely break the other scripts

Then adapt the other scripts to the new architecture
Update .gitignore: ignore new newCommands.tex file
Remove unnecessary material from lilyglyphs_common.py

I put this in the new file common.py and updated the references
to it (from the build-image-commands script).

lilyglyphs_common.py is still there but shouldn't be used anymore.
Next step must be to update the other scripts, removing any references to this file
Update email address from licence header
Scripts: Move strings from strings.py to lilyfile.py
eric4 project file
Update CHANGES.md
Work on Adapting gen-commands.py

Nearly finished, but has to be checked/tested