Fix sending wxEVT_TEXT_ENTER when using auto-completion in wxMSW

We need to explicitly generate this event from the char hook handler as
we don't get the normal WM_CHAR for it, it is apparently intercepted by
the window proc installed by the auto-completing code, so check if
wxTE_PROCESS_ENTER is used for the text entry and call a special new
MSWProcessSpecialKey() method to do the right thing if it is.

Similarly, handle Tab presses correctly if wxTE_PROCESS_TAB is used.

Closes #12613.

include/wx/msw/textctrl.h

@@ -277,6 +277,10 @@ private:
     // the simple EDIT controls
     virtual WXHWND GetEditHWND() const wxOVERRIDE { return m_hWnd; }
 
+#if wxUSE_OLE
+    virtual void MSWProcessSpecialKey(wxKeyEvent& event) wxOVERRIDE;
+#endif // wxUSE_OLE
+
     void OnKeyDown(wxKeyEvent& event);
 
     // Used by EN_MAXTEXT handler to increase the size limit (will do nothing