Weighted spreads

[MelanX]

Describe your idea

To add more randomness to islands/templates, it would be good to make the spreads a bit more random.
The idea is to allow not only spread objects in the configuration array but also weighted lists (as in loot pools). That allows to create of random structures. This could be used to place an island with a cow, or with a sheep next to the main spawn island. This is a very simple example, and obviously, this can be used for much more complicated structures like simple villages.

This idea is based on #174 and tries to implement Jigsaw-like functionality while keeping things simple.