Qt 3D: Follow Qt minor versions in import

Follow the example of most Qt QUick imports, and do automatically register an import 2.QT_MINOR_VERSION.

This allows people to always use the latest elements / properties of their target Qt version, without checking out the Qt Version / Qt Module matrix on the import to use.

It makes also our lives easier, in the sense that the latest import for qmlplugindump and in the documentation can be scripted.