Add simpler Event type inspired by simple Processing/oF style methods.

[mitchmindtree]

Currently we mostly just re-export the events emitted by the winit crate, however these can be pretty verbose and the pattern matching can get pretty grueling when all you want to do is check if a key was pressed or not.

It would be sweet if there was a function to convert WindowEvent to a much simpler event, something like:

enum SimpleWindowEvent {
    KeyPressed(Key),
    KeyReleased(Key),
    MouseMoved(Vector2),
    MouseDragged(Vector2, MouseButton),
    MousePressed(Vector2, MouseButton),
    MouseReleased(Vector2, MouseButton),
    MouseEntered(Vector2),
    MouseExited(Vector2),
    Resized(Dimensions),
}

In this case, the template could maybe look like something closer to this:

extern crate nannou;

use nannou::{App, Event, Frame};
use nannou::SimpleWindowEvent::*;

fn main() {
    nannou::run(model, update, draw);
}

struct Model {
    window: nannou::window::Id,
}

fn model(app: &App) -> Model {
    let window = app.new_window().build().unwrap();
    Model { window }
}

fn update(_app: &App, model: Model, event: Event) -> Model {
    match event {
        // Handle window events like mouse, keyboard, resize, etc here.
        Event::WindowEvent(_id, event) => match nannou::simple_window_event(event) {

            KeyPressed(key) => {
            },

            KeyReleased(key) => {
            },

            MouseMoved(pos) => {
            },

            MouseDragged(pos, button) => {
            },

            MousePressed(pos, button) => {
            },

            MouseEntered(pos) => {
            },

            MouseExited(pos) => {
            },

            Resized(pos) => {
            },

        },
        // `Update` the model here.
        Event::Update(_update) => {
        },
        _ => (),
    }
    model
}

// Draw the state of your `Model` into the given `Frame` here.
fn draw(_app: &App, model: &Model, frame: Frame) -> Frame {
    // Our app only has one window, so retrieve this part of the `Frame`. Color it grey.
    frame.window(model.window).unwrap().clear_color(0.1, 0.11, 0.12, 1.0);
    // Return the cleared frame.
    frame
}

[mitchmindtree]

I'll tackle this now 👍