allow to switch on/off at runtime a whole imported <model> via <condition>:

allow to switch on/off at runtime a whole imported <model> via <condition>:

  <model>
      <path>some/model.xml</path>
      <condition>
          <property>model/switch</property>
      </condition>
  </model>

Of course, one could add "select" animations for all <object-name> in the
<model>, but this is tedious and can hardly be done e.g. for all
objects in all instruments in $FG_ROOT/Aircraft/Instruments-3d/ etc.

The feature will be used in the bo105, so that civilian variants can
have a HSI instrument, where military variants have a TACAN etc.