CppTools: Do not call GC() more than necessary

Currently GC() is invoked if
   - 5 CppEditors were closed or the last CppEditor was closed
   - a project is about to be removed
   - a session is about to be unloaded

Thus, for the following use cases, too much GC() calls (can) happen:
    - File > Close All
    - Close All Projects and Editors
    - Changing the session

Fixed by introducing a timer.

Change-Id: I9c984d9de735fc8c6ee77a518e9fb5b63dba5881
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/modelmanagertesthelper.h

@@ -84,6 +84,7 @@ public:
     Project *createProject(const QString &name);
 
     QStringList waitForRefreshedSourceFiles();
+    void waitForFinishedGc();
 
 signals:
     void aboutToRemoveProject(ProjectExplorer::Project *project);
@@ -91,8 +92,10 @@ signals:
 
 public slots:
     void sourceFilesRefreshed(const QStringList &files);
+    void gcFinished();
 
 private:
+    bool m_gcFinished;
     bool m_refreshHappened;
     QStringList m_lastRefreshedSourceFiles;
 };