gtk/window: generate WXK_NONE key event for unknown keys

There are still plenty of X11 keys which have no WXK mapping and might be useful
for applications to act upon.

Instead of trying to remap all of them, generate a WXK_NONE key event and let
applications deal with it using GetRawKeyCode() if appropriate.

src/gtk/window.cpp

@@ -1080,12 +1080,12 @@ wxTranslateGTKKeyEventToWx(wxKeyEvent& event,
         event.m_uniChar = event.m_keyCode;
     }
 
-    // sending unknown key events doesn't really make sense
+    // sending a WXK_NONE key and let app deal with it the RawKeyCode if required
     if ( !key_code && !event.m_uniChar )
-        return false;
+        event.m_keyCode = WXK_NONE;
 #else
     if (!key_code)
-        return false;
+        event.m_keyCode = WXK_NONE;
 #endif // wxUSE_UNICODE
 
     // now fill all the other fields