The theme combo box did not show when the currently rendered item
doesn't support the service/song theme levels.
This changes the theme combo box to use the theme level from the
settings rather than the currently rendered item.
- Refactor wizard pages into external page components
- Refactor form labels into a central label with the alignment already set
- Fix some tests
- Write more tests
- Fix the theme preview generation to happen in the right place after the First Run Wizard
- Mock the settings in the DisplayWindow tests to see if that helps the Arch runner
- Add tests for the new form
