Add `isRendered` property to Views

`isRendered` is falsey before view has been rendered, and during `onBeforeRender`
`isRendered` is true when view has been rendered, and during `onRender`
`isRendered` is truthy before view has been destroyed, and during `onBeforeDestroy`
`isRendered` is false after view has been destroyed, and during `onDestroy`

CompositeView's previously used an `isRendered` property. This property has been renamed to `_isRendering`
Merge pull request #2138 from duncanbeevers/is-rendered

Add `isRendered` property to views