Some virtual functions are renamed to get rid of warnings

This MR eliminates some warnings which are only issued by GCC (v8 and v10) but not Clang (v7 and v11).

The background is that name resolution dealing with overloaded virtual member functions has some nasty pitfalls which are best avoided (hence GCC issues a warning if that is the case). To avoid possible problems two functions were simply renamed so that their names do not clash with the names from other functions (which cannot be renamed because they are inherited from Qt).