`detach` instead of `remove` old view in `LayoutView setView`

`$el.remove` also unbinds all events bound to the element, which is bad when
we're retaining the content view for reuse. This does not affect views that
expect the current behaviour of removing the element when destroying, as that
is still done by `_removeChild` if the retain count of the view being removed
is 0.

https://github.com/walmartlabs/thorax/issues/332