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/debugger/debuggermanager.h

@@ -311,7 +311,7 @@ public slots:
     void updateWatchData(const WatchData &data);
 
     void sessionLoaded();
-    void sessionUnloaded();
+    void aboutToUnloadSession();
     void aboutToSaveSession();
 
     void assignValueInDebugger();