[about-distro] update osrelease parser for latest values in spec

Summary:
and add a simple unit test so one can refactor without fear of regressing
something :((

- osrelease now takes an optional filepath as ctor argument to enable
  dummy data for testing
- instead of exhaustively if-deffing the config keys are now mapped to
  member variables in a hash and then resolved through that. this makes
  the mapping a bit more readable but most importantly reduces the
  syntactic cruft around it so as to make everything easier to copy paste
  without messing up by not seeing the important elements clearly
- all currently supported values are now under test

among the newly supported keys is LOGO which we'll be able to use for
our logo \o/

Reviewers: apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19160