Pass model as an argument to triggered events in the selectable mixin

Pass a model as an argument to triggered events in the selectable mixin. To possible get reference on the model when listening events of the collection.


Before:
```js
collection.on("selected", function() {
  // here we don't know which a model is related with this event
});
```

After:
```js
collection.on("selected", function(model) {
  // here we can get related model for this event
});
```