Implement CSS z-index prop in GTK CSS
GTK would benefit from having a native way to order components based on the z-axis, instead of doing workarounds.
Since this issue aims to ask to have a behaviour like CSS's, it'd be ordering components two-way by using GTK CSS: Using only increased negative values (-1, -2,..) means that the order is tails-to-head, and only positive (1, 2,..) means head-to-tail. With 0, or omitting the CSS prop z-index, being the "ground level" or no z-axis usage.