Add `Trigger` trait

[bvssvni]

pub trait Trigger<U> {
    /// Updates state and returns true if triggering on event.
    fn trigger(&mut self, event: &U) -> bool;
}

This will make it possible to handle arbitrary mappings with Box<Trigger>. For example, in Piston we have a FirstPerson camera controller which uses FirstPersonSettings. The key controls are limited to the keyboard. By using Box<Trigger> instead we can for example control the camera with both keyboard and gamepad at the same time.

[bvssvni]

This could be moved to rust-event.

[bvssvni]

Will wait with this since we now can use the Button enum.