BUG: 336020 Fix updating function signature for python scripts to reflect object selection

BUG: 336020 Bug fix for: https://bugs.kde.org/show_bug.cgi?id=336020

Only change the function definition, not the rest of the text, so that the user modifications to the function body would be preserved. The function definition gets updated every time the script page of the wizard is accessed.

Please comment on code style, I have used brace initialisation and auto but I am not sure if that is allowed. All the code I was checking was 15+ years old, so C++11 feature weren't introduced yet.