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/cmakeprojectmanager/cmakeproject.h

@@ -83,7 +83,7 @@ public:
     ~CMakeProject();
 
     QString displayName() const;
-    QString id() const;
+    Core::Id id() const;
     Core::IDocument *document() const;
     CMakeManager *projectManager() const;