Applying a template to itself should be illegal - currently results in js stack overflow

[Charles Goodwin]

If a user accidentally applies a template to itself we should probably give an explicity error stating it is illegal.

Currently if you apply a template to itself, running Vexi gives output like the following:
         Scheduler: A JavaScript thread spawned with vexi.thread threw an exception:
                    JSExn: stack overflow
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        at org.vexi.lib.widget.separator:8(apply)
                        ... and 492 more