Add a id() method to projects

* This makes it easier to distinguish projects in the
   userfileaccessor

src/plugins/genericprojectmanager/genericproject.h

@@ -70,6 +70,7 @@ public:
     QString configFileName() const;
 
     virtual QString displayName() const;
+    virtual QString id() const;
     virtual Core::IFile *file() const;
     virtual ProjectExplorer::IBuildConfigurationFactory *buildConfigurationFactory() const;
     virtual ProjectExplorer::IProjectManager *projectManager() const;