sessionmanager: replace signal sessionUnloaded() by aboutToUnloadSession()

sessionUnloaded was sometimes emitted when the session manager was in
an unsafe state (between delete m_file and  m_file = new ...) leading
to crashes when slots connected to the signal accessed the sessionmanager
again. This patch moves the signal emission before  delete m_file.

Reviewed-by: dt

src/plugins/cpptools/cppmodelmanager.h

@@ -118,7 +118,7 @@ private Q_SLOTS:
     // this should be executed in the GUI thread.
     void onDocumentUpdated(CPlusPlus::Document::Ptr doc);
     void onAboutToRemoveProject(ProjectExplorer::Project *project);
-    void onSessionUnloaded();
+    void onAboutToUnloadSession();
     void onProjectAdded(ProjectExplorer::Project *project);
     void postEditorUpdate();
     void updateEditorSelections();