This is enabled by doing "Item.layer.enabled: true". The implementation
is solely based on the existing shader effect (source) and simply
swaps in a sibling next to the item when enabled.

This change also adds the QSGTextureProvider to the public API,
as it is now a natural part of the QQuickItem API since all items
can be textures.

Change-Id: I26705c11e92d5623a5121300acc123782b784077
Reviewed-by: Kim M. Kalland <kim.kalland@nokia.com>