qmljs: add infrastructure handling qml dialects better

QmlBundles enables us to treat the different qml
dialects differently.

Add imports completion.

Change-log: [Qml/JS Support] Corrected handling of QtQuick2 only features.
Change-log: [Qml/JS Support] Added import completion in editor.
Task-number: QTCREATORBUG-8750
Task-number: QTCREATORBUG-8624
Task-number: QTCREATORBUG-8584
Task-number: QTCREATORBUG-8583
Task-number: QTCREATORBUG-8429

Change-Id: I1384b1b23136a85b4d077895ea86f92960da9e71
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/libs/qmljs/qmljsdocument.h

@@ -78,6 +78,7 @@ public:
 
     bool isQmlDocument() const;
     Language language() const;
+    void setLanguage(Language l);
 
     AST::UiProgram *qmlProgram() const;
     AST::Program *jsProgram() const;