DolphinWX: Exit after closing main window or on signal

Previously Dolphin would only exit if the main window is closed,
and Confirm on Stop is enabled.

This makes Dolphin's behaviour more consistent by always exiting
if the main window is closed or on shutdown signal.

Source/Core/DolphinWX/Frame.cpp

@@ -615,7 +615,6 @@ void CFrame::OnClose(wxCloseEvent& event)
       event.Veto();
     }
     // Tell OnStopped to resubmit the Close event
-    if (m_confirmStop)
     m_bClosing = true;
     return;
   }
@@ -1716,6 +1715,7 @@ void CFrame::HandleSignal(wxTimerEvent& event)
 {
   if (!s_shutdown_signal_received.TestAndClear())
     return;
+  m_bClosing = true;
   Close();
 }