World generation refactor

[ceeac]

This patch moves the world generation code out of karte_t; as a result, the amount of code in simworld.cc is reduced by nearly 20%. While I have done found no issues during testing, it may be a good idea to integrate this only after the next release, depending on when the next release is.

[prissi]

I think one should discuss how to proceed with such reorganisation patches.

I confer that broken code makes future contributions easier. On the other hand, the code between extended and OTPR has already drifted so far apart that patches became extremely tedious work, and reduces the chances of mutual contribution even further.

As for release, I am on holiday, and for the next week likely without notable internet access (on a sailship). Hence, the next release is likely rather the end of August.