FR: Enhanced Event Sexp Handling in QtFRED

In FRED, operators like `is-event-true-delay` that reference events can only right-click->events see events that existed before the current instance of the editor dialog was opened. This is probably because the sexp tree that gets the list of events pulls from the global and not the local working copy. Switching that would be trivial with an override in QtFRED (That's what Messages does). What needs to be checked and handled is what happens if an operator references an event that exists in the working copy but is deleted before the working copy is saved. This is solvable with a quick pass over the tree for those situations and may actually be already handled but testing will need to be done to verify.