--xclass switch actually changes WM_CLASS property, but openbox uses
_OB_APP_CLASS for matching. This property is created together to the original.
Probably roxterm's --xclass switch, changes WM_CLASS after window mapping,
making _OB_APP_CLASS useless. This behaviour seems to be non-compliant with
icccm spec http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.2.5
Is it possible change switch behaviour?