First implementation of "why not" feature

The inspector now allows users to type an event into a dialog box; upon
doing so each node in the tree is annotated by either a tick or a cross
icon to indicate whether or not the event is offered. Response is
immediate to maximize user friendliness.
"Why not" system completed.

Now in the inspector the user is given a reason appropriate to the
selected component stating why an event is not offered, if it is indeed
not offered.