Urgent transition missing check

[Jiri Srba]

Open the intro example and double click on the transition T1. It is not possible to make it urgent in the dialog as the check that is run finds out that the input arc has a nontrivial input interval (for urgent transitions we only allow [0,inf) on input arcs). This is a correct behaviour, however, if you just select the transition T1 and press U (toggle the urgent transition) it will allow to make it urgent as this toggle action does to execute the check if it is possible to make the transition urgent. The same check as in the dialog should be run before turning the transition into urgent.