API: Clean up KReportUnit
- remove KReportUnit::ListOption (T4967), add Invalid type, better support descriptions and symbols
- mark default constants as extern to avoid copying, add DEFAULT_UNIT_TYPE
Build KReport with tests enabled, run tests, run KReportExample, try the Unit combo for the page units.
(switching units for page is still broken but this is not related to this very patch)
Maniphest Tasks: T4967
Differential Revision: https://phabricator.kde.org/D6756