QmlDocument: QML language support (like for C++)

[narnaud]

It would be nice to be able to create Query and extract symbols from QML documents.

There are multiple steps for that:

• Integrate QML parser for TreeSitter (most likely requires a 3rd party)
• Make QmlDocument a CodeDocument sub-class
  => you should see symbols in Knut Gui
• Add some specific API to extract an item with its properties
  => would be nice to easily do some introspection, without using TreeSitter query (they are used internally)