[Pharo-project] Explicit ordering of submorphs

Currently, the submorph order isn't explicit. Which means that adding several submorphs in a certain order, is no guarantee that they will appear in the desired order when laid out using a LayoutPolicy. Because the #morphicLayoutNumber is used. I've propose the following: 1. Add Morph>>precedence 2. Add Morph>>orderedSubmorphs 3. Modify the existing LayoutPolicies to use orderedSubmorphs instead of submorphsDo: ISSUE 3136.