Establish a language versioning mechanism for QML

In order to manage differing behavior between QML version 2 and version 3 we need a way to declare which version of the language a particular file should use. The following options should be provided:

1. A "pragma version {2|3}" in the header of the QML file. This has the highest priority
2. The file ending. "qml" weakly hints to version 2, "qml3" forces version 3. A file ending in "qml3" with "pragma version 2" in the header should be a compile error.
3. Failing to detect any of the above, Qt5 will assume version 2 and Qt6 will assume version 3. This should produce a warning, already in Qt5.