Include p2 director in Toolbox installation to be able to install

additionally features (p2 IUs) from the command line.

[Feature][Toolbox]

org.lamport.tla.toolbox.feature.standalone/feature.xml

@@ -262,4 +262,18 @@
          fragment="true"
          unpack="false"/>
 
+   <plugin
+         id="org.eclipse.equinox.p2.director"
+         download-size="0"
+         install-size="0"
+         version="0.0.0"
+         unpack="false"/>
+
+   <plugin
+         id="org.eclipse.equinox.p2.director.app"
+         download-size="0"
+         install-size="0"
+         version="0.0.0"
+         unpack="false"/>
+
 </feature>