Add a id() method to projects

* This makes it easier to distinguish projects in the
   userfileaccessor

src/plugins/cmakeprojectmanager/cmakeproject.h

@@ -75,6 +75,7 @@ public:
     CMakeBuildConfiguration *activeCMakeBuildConfiguration() const;
 
     virtual QString displayName() const;
+    virtual QString id() const;
     virtual Core::IFile *file() const;
     virtual ProjectExplorer::IBuildConfigurationFactory *buildConfigurationFactory() const;
     virtual CMakeManager *projectManager() const;