Allow dismantling of buildings

It is only possible to destroy buildings in widelands. Stored wares and all construction materials are lost.

Implement a destructionsite as the opposite of the constructionsite. All workers leave the building. Then a builder comes and first carries out all remaining wares. Than start dismantling the building. Part of the construction materials are regained in this process.

It should still be possible to destroy buildings as it is possible now. The player can decide between destroying (imediately, no worker needed) and dismantling (takes a while and needs a worker).

Not all materials should be regained. Only the refined ones should. How to configure that?
1. In the conf file of the building
2. A "regain factor" for every building material

I think the second way (adding a factor to each material) is the best way.