rs_actionmodifycut.h
I should be paid as much as my dentist - I am spending almost as much time pulling teeth here. What Qt class is "RS_ActionModifyCut" ultimately derived from?

Look, it is basic C++: any QWidget-based class takes an optional pointer to its parent QWidget as an argument to its constructor. This includes QDialog and classes derived from QDialog. If you try to pass in a pointer to a class that is not derived from QWidget, then you will get a compile-time error. Passing "0" always works, because a zero can be cast to a pointer of any type. Passing "this" will work only if the class that "this" is an instance of is derived ultimately from QWidget.