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.cpp

@@ -1046,7 +1046,7 @@ void DebuggerManager::sessionLoaded()
     loadSessionData();
 }
 
-void DebuggerManager::sessionUnloaded()
+void DebuggerManager::aboutToUnloadSession()
 {
     cleanupViews();
     if (m_engine)