Resolves#3037
This is a do-over of c85aa11, with the following differences:
- Fixed `_ASTableViewCell` properties **always** being mutated to match corresponding `ASCellNode` passthrough properties, even when the hosted `ASCellNode` is currently `nil`. Now only inheriting passthrough properties when there’s an actual instance of `ASCellNode` to read them from.
- Corrected spelling of `ASCellNode`’s `separatorInset` property.
- Reverted `_ASCollectionViewCell` inheriting the hosted cell node’s `backgroundColor` & `clipsToBounds` properties. This seems to be surprising and unwanted behavior to some, as seen in #3053 and #3044.
- Moved passthrough of `UITableViewCell`’s `selectedBackgroundView` into `_ASTableViewCell`’s `-setNode:` for consistency.
