DolphinWX: Shut down cleanly on signal

This makes DolphinWX shut down cleanly, just like it would with
File->Exit when it receives a SIGINT, SIGTERM (Unix) or some signals
on Windows.

The default signal handler will be restored after a first shutdown
signal so a second signal will exit Dolphin forcefully.

Source/Core/DolphinWX/Frame.h

@@ -186,6 +186,7 @@ private:
   };
 
   wxTimer m_poll_hotkey_timer;
+  wxTimer m_handle_signal_timer;
 
   wxBitmap m_Bitmaps[EToolbar_Max];
 
@@ -328,6 +329,7 @@ private:
 
   void PollHotkeys(wxTimerEvent&);
   void ParseHotkeys();
+  void HandleSignal(wxTimerEvent&);
 
   bool InitControllers();