Use Core::Id in project-related objects

Use Core::Id for all the project related objects in favor of plain
QStrings.

Change-Id: I790ab40cb29899efdb49c413a77609486f52e683
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/qmlprojectmanager/qmlproject.h

@@ -69,7 +69,7 @@ public:
     QString filesFileName() const;
 
     QString displayName() const;
-    QString id() const;
+    Core::Id id() const;
     Core::IDocument *document() const;
     ProjectExplorer::IProjectManager *projectManager() const;
     Internal::QmlProjectTarget *activeTarget() const;