Make ufo invisible over MP again

Before fallback models were introduced, the ufo
was invisible to other MP pilots that were not in a
ufo themselves.

When fallback models were introduced, at long range
the ufo was replaced by the glider model, and so became
visible.

This change makes it invisible again by using the ufo as its
own fallback model.  This is safe because the ufo model is
very lightweight and also shipped with fgdata.

AI/Aircraft/fallback_models.xml

@@ -16,14 +16,18 @@
   0       Default.  Will also be used if an AI model is not found
   1-50   Single engine GA aircraft
   51-100 Multi-engine GA aircraft
-  101-150 Turboprop airliners
-  151-200 Twin engined narrow body jet airliners
-  201-250 Twin engined wide body jet airliners
-  251-300 Four engined jet airliners
-  301-350 Military combat aircraft
-  351-400 Military non-combat aircraft (tankers, transports)
-  401-500 ????
-
+  101-150 Business Jets
+  151-200 Turboprop airliners
+  201-250 Twin engined narrow body jet airliners
+  251-300 Twin engined wide body jet airliners
+  301-350 Four engined jet narrow body airliners
+  351-400 Four engined jet wide body airliners
+  401-450 Single engine prop military aircraft
+  451-500 Multi-engine prop military aircraft
+  501-550 Jet combat aircraft
+  551-600 Military large aircraft (tankers, transports)
+  601-700 Helicopters
+  701-800 Other
 -->
 <PropertyList>
 
@@ -172,4 +176,9 @@
 <model n="710" type="string">AI/Aircraft/G-164/Models/G-164A-ai.xml</model>
 <model n="711" type="string">AI/Aircraft/Gee-Bee/Models/geebee-ai.xml</model>
 
+<!-- Special case as the ufo is always installed and light enough not to need a separate
+AI model.  We also want to load it in preference to the default glider because it will
+then be invisible to other MP players who are not in a UFO themselves. -->
+<model n="712" type="string">Aircraft/ufo/Models/ufo.xml</model>
+
 </PropertyList>

Aircraft/ufo/ufo-set.xml

@@ -78,7 +78,7 @@
                 <position-z-m type="float">0</position-z-m>
                 <jetway-hood-deg type="float">3</jetway-hood-deg>
             </door>
-
+			<fallback-model-index>712</fallback-model-index>
 		</model>
 
 		<instrumentation>