Feature: Background image

- Note: Only mininum tests are done, since I don't have enough time
  right now. Expect bugs.

- Introduces background image support, with PNG/JPEG/GIF reading
  capability. (libpng/libjpeg/giflib) Syntax is explained in
  skippy-xd.rc-default .

- Introduces a certain degree of support for non-EWMH/GNOME compliant
  WMs. (#14, #20)

- Include window decoration in thumbnails if requested. (#13)

- (Hopefully) fixes high CPU usage. (#17)