View: set isDestroyed to false by default
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 #2167 from megawac/isDestroyed-bool

View: set isDestroyed to false by default
Fix #2134; LayoutView should remove its element before destroying child regions
LayoutView: add a destroyImmediate flag to enable fast cleanup