Resolve "Always use provided Catch2 and trompeloeil"

Move all external deps to deps folder.

TODO:

• need to update internal 3rdparty folder, with removed Catch2 and trompeloeil

Closes #362 (closed)

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()