ECMAddQtDesignerPlugin: fix support for Qt6
Needs to be rebased on !273 (merged) to fully work, completing the changes done there to use QT_MAJOR_VERSION with all Qt macro/oackage names.
@vkrause @cgiboudeaux @ahmadsamir
Needs to be rebased on !273 (merged) to fully work, completing the changes done there to use QT_MAJOR_VERSION with all Qt macro/oackage names.
@vkrause @cgiboudeaux @ahmadsamir