When you register your handler using org.eclipse.ui.handlers, you can
create an <enabledWhen/> expression. The default variable is a
Collection with the current selection, and the current selection
ISelection is provided with the variable "selection". You can feed both
the default variable and the selection variable to <iterate/>
The IHandler can also control its enabled state programmatically,
returning it from isEnabled() and firing a HandlerChangeEvent when the
enabled state changes ... but a handler is only asked after it has been
loaded.