[Qt 6] Fix warnings when built (QML module) against Qt 6 (Follow-up from "Port QML module to Qt 6")

The following discussion from !42 (merged) should be addressed:

• @sunweaver started a discussion:

  Please add an issue that requests Qt6 build to build with -DENABLE_WERROR=ON. At some point it will be ok if the Qt5 build throws warning, but the Qt6 build should run through smoothly.