CppTools: Do not garbage collect files in the working copy

...except the configuration file if no projects are open. For this case
there is no need to keep the configuration file around.

Task-number: QTCREATORBUG-9829

Change-Id: I51b01b30c17cbc1ced491ef2c47c338dae6ed983
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpptoolsplugin.h

@@ -195,6 +195,7 @@ private slots:
     void test_modelmanager_snapshot_after_two_projects();
     void test_modelmanager_extraeditorsupport_uiFiles();
     void test_modelmanager_gc_if_last_cppeditor_closed();
+    void test_modelmanager_dont_gc_opened_files();
 
 private:
     void test_completion();