Improved the select example. (#501)

Now the `OnEventView.on_pre_event_inner()` calls return
`Some(EventResult::Consumed(Some(Callback)))` instead of
`Some(EventResult::Consumed(None))`.
This follows the guidelines from documentation of methods returning a
`Callback`, which say that it should be ran on the `Cursive`.
While in this example this doesn't make a difference, the previous
version created confusion for new users who might not realize you can
pass the `Callback`s to the `Cursive` this way.

examples/src/bin/select.rs

@@ -27,12 +27,12 @@ fn main() {
     // Let's override the `j` and `k` keys for navigation
     let select = OnEventView::new(select)
         .on_pre_event_inner('k', |s, _| {
-            s.select_up(1);
-            Some(EventResult::Consumed(None))
+            let cb = s.select_up(1);
+            Some(EventResult::Consumed(Some(cb)))
         })
         .on_pre_event_inner('j', |s, _| {
-            s.select_down(1);
-            Some(EventResult::Consumed(None))
+            let cb = s.select_down(1);
+            Some(EventResult::Consumed(Some(cb)))
         });
 
     let mut siv = cursive::default();