Allow Grouping of Similar Objects

[egocarib]

Several users have requested the option to have different markers for each type of wild crop (turnips vs. rye, etcetera).

One elegant way to implement this would be to define grouped settings. This would also be a nice way to handle the different types of traders. If a user wants only to configure the group with a single shared setting (i.e. all crops, or all traders) they can do that. But the UI should also allow them to "expand" the group and define custom markers for everything within the group if desired.

This will keep settings simple by default, but allow more customization for those who want it.