ENH: configure outputModel selector of CombineModels

Author: mauigna06

CombineModels/Resources/UI/CombineModels.ui

@@ -180,7 +180,7 @@
          <bool>true</bool>
         </property>
         <property name="addEnabled">
-         <bool>false</bool>
+         <bool>true</bool>
         </property>
         <property name="removeEnabled">
          <bool>true</bool>
@@ -420,4 +420,4 @@
    </hints>
   </connection>
  </connections>
-</ui>
+</ui>