* ResourceType trait has been removed.
* Simulation-related data is now defined on the
ResourceLayer (which mods can subclass/replace).
* Support non-money resources by moving the resource
values to the PlayerResources trait.
* Allow mods to disable the neighbour density override
and instead always use the map-defined densities.
* Allow mods to define their own resource placement
logic (e.g. allow resources on slopes) by subclassing
(Editor)ResourceLayer.
* Improve ability to subclass/override ResourceRenderer
by exposing more virtual methods.
