Add variable use cases to model sample

Author: Tewan

samples/model_example.qc

@@ -18,4 +18,8 @@ $collisionmodel "example_phys.dmx"
 
 // This model's materials are relative to these paths. Keep / as a fallback just in case
 $cdmaterials "/"
-$cdmaterials "models/props_examples/"
+$cdmaterials "models/props_examples/"
+
+$definevariable Test "test"
+$model $Test$
+$model -$Test$