Intelligent worker slot reordering

[Noordfrees]

https://www.widelands.org/forum/topic/4983
Internally, every slot is assigned a minimum worker rank at building creation time. One slot for a miner or better, one for a chief miner or better, and one for a master miner or better. That's the same for every building that needs experienced workers. No reordering is possible currently, +1 for implementing intelligent detection whether the slot requirements for a filled slot and a vacant slot could be swapped.