Summary:
previously we'd ignore unknown keys. in preparation of moving to a library
let's instead collect them so a user of the class may also access them.

- new extraKeys helper and extraValue getters
- extras are always processed as String
- test case fixture extended + now has comment variants (previously
  comments couldn't be tested since they were skipped as either comments
  or invalid/unknown lines. with the extras API we can actually verify
  that they do not incorrectly end up getting parsed

Test Plan: test still passes

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19976