Feature: register new lua function DrawBuildSquare to show build squares on custom building commands

[aurelienlt]

Registers canBuildEach = Spring.DrawBuildSquare(unitIds, buidCommands) which returns if each building command is allowed, while displaying the usual grid.

Mostly forking CUnitDrawerLegacy::ShowUnitBuildSquare and reproduce GuiHandler's behavior with overlapping queue.

My first MR on that project, so please lemme know best practices.

[sprunk]

I think drawing should not be doing any validity calculations (add another function that checks overlap, buildability or whatnot) and should draw a single blueprint.