#169 Mouse Button Mask doesn't work for TranslatingGraphMousePlug

[mulova]

When I replaced
translatingPlugin = new TranslatingGraphMousePlugin(InputEvent.BUTTON1_MASK);
with
translatingPlugin = new TranslatingGraphMousePlugin(InputEvent.BUTTON3_MASK);
It doesn't work.

If AbstractGraphMousePlugin.checkModifiers() is changed like below, right mouse dragging works.
public boolean checkModifiers(MouseEvent e) {
return (e.getModifiersEx() & modifiers) != 0;
}

To do this, InputEvent.BUTTONx_MASK should be changed to InputEvent.BUTTONx_DOWN_MASK.