Check whether a proFile exists before using isDir

• Fix for autosave; however this problem might exist in other places as well (e.g. asking whether the user want to save the project when opening one; despite no old project being loaded)

Problem is that QFileInfo::isDir gives a false if the given path does not exist, leading to us thinking it is a valid pro-file currently loaded.

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

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()