Fix WorldObjects not loading. Fixes previous commit.

Refixes #187.

(GlobalObjects worked fine.)
Merge remote-tracking branch 'origin/master' into BetterRivers
Add RiverCustomHeightControl. Add comments.

- Rename RandomRivers to ImprovedRivers. Doesn't autoconvert.
- CustomHeightControl (and RiverCustomHeightControl too) won't clear
itself anymore when the number of elements is incorrect. Instead, it
will simply keep what fits. Should save a few headaches.