Added listening changes alpha of side items.

[b3tman]

This allow to change alpha value for side items. If sideItemAlpha = 0, then left and right not centered items has alpha = 0. It allows to change alpha for some elements in the cell.

Example. func didChangeAlpha(_ alpha: CGFloat, for indexPath: IndexPath) { guard let cell = collectionView.cellForItem(at: indexPath) as? MagazineCell else { return } cell.setTextContentAlpha(alpha) } }

func setTextContentAlpha(_ alpha: CGFloat) { titleLabel.alpha = alpha descriptionLabel.alpha = alpha timingView.alpha = alpha freeView.alpha = alpha freeLabel.alpha = alpha iconImageView.alpha = alpha iconBackgroundImageView.alpha = alpha }