Add test for pet-file

• Add regression test for checking whether 00_empty.pet gets saved (mostly) exactly as it is now
• Also fixed error in reading in FROM_COLOR in colorMarkerWidget.cpp (assigned saturation to value)

For motivation, see issue #371 (closed)

Closes #371 (closed)

Reviewer Checklist

Formatting

• the pre-build checks succeed

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

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()