Add more thread safety to ModelManagerInterface

Add ModelManagerInterface *ModelManagerInterface::instanceForFuture()
method. If the returned instance is not null, it's guaranteed
that it will be valid at least as long as the passed QFuture object
isn't finished.

Use instanceForFuture() in Link c'tor, as it's called from
non-gui thread.

Change-Id: I7e5ee6ad27e8f71cc0cef7fd9a91b710e2f8f662
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@qt.io>

src/libs/qmljs/qmljsmodelmanagerinterface.h

@@ -126,6 +126,7 @@ public:
     static Dialect guessLanguageOfFile(const QString &fileName);
     static QStringList globPatternsForLanguages(const QList<Dialect> &languages);
     static ModelManagerInterface *instance();
+    static ModelManagerInterface *instanceForFuture(const QFuture<void> &future);
     static void writeWarning(const QString &msg);
     static WorkingCopy workingCopy();