Many fixes, improvements and simplifications to existing code.

Implements the first part of the changes discussed in
  https://gitlab.com/inkscape/inkscape/issues/333

I hereby disclaim any implicit or explicit ownership of my changes in this
changeset, and put them under a multiple licence consisting of your choice of
one of more of:

- The CC0 / Public Domain - https://creativecommons.org/choose/zero/ .

- The MIT / Expat license - https://en.wikipedia.org/wiki/MIT_License

- The default licence of your project

- The https://en.wikipedia.org/wiki/GNU_Lesser_General_Public_License - version
2.1 or higher

- The https://en.wikipedia.org/wiki/GNU_General_Public_License - version 2 or
higher

- Any licence in the 2018-Aug-27 popular licenses list of
https://opensource.org/licenses

- The https://en.wikipedia.org/wiki/Apache_License version 2.0 or later

- The https://en.wikipedia.org/wiki/Artistic_License version 2.0 or later

- The https://en.wikipedia.org/wiki/ISC_license

- The https://opensource.org/licenses/BSD-2-Clause

Crediting me will be nice, but not mandatory, and you can change the licence
of the project without needing my permission.

- add license headers to everything
- convert a few files from public domain or LGPL2.1+ to GPL2+
- some archaeology to clarify which files are from which library