macOS: Add native interface API to manage visual effect view

NSVisualEffectView is the only supported way to add effects to
views, such as showing a blurred representation of the content
under the window, or within the window.

Now that we render the content of our QNSView into a sublayer
of the view's root layer, we can add NSVisualEffectViews as
child views without obscuring the view's own content, by making
sure that the NSVisualEffectView layer has a lower Z order in
the layer tree of our view's root layer. This works because
adding a layer-backed or -hosted view as a subview will also
add its layer as a sublayer of the view's layer, making the
effect layer and our content layer sibling layers.

Change-Id: Iab822e8462f54025559b3e3f26c7df668c885d75
Reviewed-by: Richard Moe Gustavsen <[email protected]>

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