Resolve "Check whether mProFile is a directory"

Change check if mProFileName is not a directory to checking if it is a file. This prevents errors that a non-existent directory is treated as an existing file.

In case of an invalid path (neither file nor directory), this value is reset to an empty string.

closes #353 (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()