[FEATURE] Fallback Event Handler for unhandled events

**Is your feature request related to a problem? Please describe.**

I'm needing to pass the character and key codes up to another processing layer if nothing in the cursive stack handled them, however it seems the primary way to do that is via `add_global_callback` and `set_on_post_event` and such calls on the `CursiveRunner` and such objects. The `add_global_callback` just delegates to `set_on_post_event` after changing the `Event` type to an `EventTrigger` type. If I implement `EventTrigger` directly and call `set_on_post_event` with it then I'm able to get the callback called for every event, however I don't know what event actually caused it as the callback only takes a `&mut Cursive`. Looking inside `set_on_post_event` shows that it just calls `self.root.set_on_event(trigger, crate::immut1!(cb));` in the `Cursive` object, where `self.root` is the `RootNode` (an `views::OnEventView` type), and the `OnEventView::set_on_event` call just ends up delegating to `OnEventView::set_on_event_inner`, of which it like all the above are `pub`. The `OnEventView::set_on_event_inner` call is different than the prior in that it takes a callback that takes both a `&mut T, &Event` where `T` is the `views::ScreensView` that is passed in to the `OnEventView::init`.

In short it looks like all the information is there to be able to call a callback that takes a `(&mut Cursive, &Event)` (and perhaps the `T` view as well), but not finding any way to do so on the `root` node.

**Describe the solution you'd like**

Optimally a way to register a fallback event handler that gets passed events that were not handled from within the Cursive view stack that passes at least the `&mut Cursive` and `&Event` that caused it (and perhaps the root view but that seems needless for me though could be useful in general) so we can handle things like a `Char(_)` for any character so we can handle unhandled events for other purposes.

**Describe alternatives you've considered**

I've tried looking for callbacks that can register both a `&mut Cursive` and an `&Event` on the root node. Tried looking to see if anything else is called if the root node doesn't handle an event, etc... etc... Nothing found. Tried looking for ways to acquire access to the `root` view via alternative methods, other event hooks that I could manually trigger, etc... Having difficulty finding any way through.

**Additional context**

In this case I'm wanting to pass unhandled character's, modifiers of characters, and all other possible keypresses up to another library after translating the types so it can handle the inputs for things that Cursive itself did not explicitly handle.