Disable main window when showing msgbox in wxSafeShowMessage()

Pass correct parent HWND to ::MessageBox() in order to disable the
window while the message box is shown, as this function is supposed to
be similar to modal wxMessageBox() and it was unexpected that the
application could be reentered via the event handlers from inside it.

This required adding wxAppTraits::GetMainHWND() in order to only use the
HWND in GUI applications from the function defined in non-GUI code.

include/wx/msw/apptbase.h

@@ -58,6 +58,9 @@ public:
     // write text to the console, return true if ok or false on error
     virtual bool WriteToStderr(const wxString& text) = 0;
 
+    // return the main application window or 0 if none
+    virtual WXHWND GetMainHWND() const = 0;
+
 protected:
 #if wxUSE_THREADS
     // implementation of WaitForThread() for the console applications which is