qt6 support

Recent marble versions are requiring qt6 libs. I tried fixing the compilation error related to that in ztgps (mainly qvector to qlist to interact with marble), but even though linking works, I get a segfault at startup. I'm assuming it's because ldutils is using qt5 versions of the rest of the qt libs, and somehow qt5 & qt6 are incompatible with each other (did not debug further). Thus, is there any plan to support marble-qt6 in the future ?