Move some snippets, new category-listing tool

I *did* it (is that now good or bad, given the time I spent on it?):
I wrote an initial version of a script that will be used to auto-generate documentation from `openlilylb` :-)

It is a PyQt application, and I don't know the requirements to run it. But you can be quite sure that if you have Frescobaldi running then this script will work as well.

It is located in the `/docgen` folder and can simply be run from there with `python olldoc.py`
Currently it will do the following:

- walk over `/library/oll` and load the snippets defined there
- walk over `/usage-examples` and load the example files loaded there.
- parse the snippets for categories and tags in use
  (with a *very* hacky appraoch - this is just for now)
- output a summary in `/doc/categories.txt`

To test it I have moved two more of my snippets to the new location(s) and renamed them appropriately, but didn't start with cleaning them up. The idea was first to move all files and then reorganize them using the information gathered by this script.

What it currently does is quite hacky, but I hope I have built it on a sufficiently clean foundation so we can go ahead from here.