Change default export option to "use meter" only and set `fix` to true

Following the issue #434 (closed), all export options are disabled by default, except use meters.
Additionally, everything that has a fix checkbox should now have this checkbox enabled by default. Crucially, this includes the coordinate system.

Reviewer Checklist

General code quality

• naming conventions are met (see .clang-tidy for detailed information)
• no static analyzer warnings in new code parts (e.g., use clang-tidy for checking)

General usability

• old versions of pet-files are still loadable
• documentation updated (if applicable)

Only if changes in UI

• new elements are also saved and loaded from pet-file
• check if tab order is still correct
• all new SpinBoxes are promoted
• new keybindings added to Petrack::keyBindings()

Closes #434 (closed)