Towards multilayer walls in FreeCAD

I think I found a quite elegant solution for multilayer walls. It is by no means a complete feature, but I think it gives us a very interesting start on the topic.

Basically I added an "Offset" property to walls, so they can be offsetted from their baseline. With this, several walls can be based on the same baseline, resulting in a multilayer wall:

A window can be added as usual, drawn on one of the faces of any of the "sibling" walls:

Upon creation, the window not only creates a hole in its base wall, but also in all the sibling walls (= walls based on the same baseline)

There are of course different issues to solve later such as what happens when a wall intersects a multi-wall, but for that we need more complete materials support, because everything will be decided depending on the material. I will also make the hole system better, allowing the user to add a custom shape to define the hole, so one can do more complex things such as having a different hole size for each of the layers of a multilayer wall.