Minor log message changes
Refactor: begin collecting little algorithms

Basic geometric and numeric stuff belongs in lib/.
Backgrounds: be less fussy, use PyGI syntax.

* Allow JPEG images to be used in the background library
* Walk background library directories recursively to find candidates
* Permit background images to be used which have alpha
  - works by compositing onto a black background
* Allow images that aren't a strict multiple of the tile size
  - works by tiling and repetition up to a preset limit
  - if that hits the lcm, that's great!
  - otherwise, scale closest fit encountered down to nearest tile multiple
  - hopefully still reasonably pretty and close to the original aspect
* Project: update copyright
* Coding: use PyGI syntax
* Coding: various stylistic fixes
* UI: tweak display of the background previews
Default bg: use fallback image, no dark-theme test

Use a nice fallback image if default.png doesn't exist. We don't ship a
standard default.png now.

No longer test whether MyPaint is configured to use dark themes to decide
the default background. Not every theme will have a "dark" variant, and
some themes are dark by default. Better to not try to be too clever, and
pick a fallback colour and fallback image that works OK for both dark and
light surrounds.
Ripped out legacy Psyco code

Psyco has been unmaintained for years and in all likelihood doesn't even
work with Mypaint, thanks to swig. In the unlikely case Psyco was
installed, Mypaint would probably just fail to start.