#12 Continue building of structures when the first one isn't placed yet

[Danau]

This patch (on top of the latest master branch, v0.97 + fixes from https://sourceforge.net/p/dunelegacy/patches/11/) implements the option to construct new structures in a construction yard before the first one is placed.
This allows human players to use the build queueing option to queue a set of buildings and then place them all in one go.

AI players typically don't use this feature, because they place constructed buildings right after they are finished, but this will help human players that want to build large sections of walls or turrets or any other large base without continuously waiting for buildings to finish. (You just queue all buildings once and then place buildings whenever you have time.)

Part of this feature is related to https://sourceforge.net/p/dunelegacy/feature-requests/41/ in that it becomes possible to lay a path of slabs without the waiting time by queueing all slabs and then placing them all when they are all finished constructing.