Terrain feature precedence

Currently when applying terrain features such as areas or modifiers there's no deterministic order to how they should be applied if two or more overlap. This is problematic because the client often receives sights of the terrain features in stochastic order.
We therefore need to add some kind of precedence system so that this becomes deterministic.
The proposed solution is to by default use the entity id (being an int) to sort the features. Higher entity ids gets precedence to lower ones. This would fit with the common usage where a newly created feature would "overwrite" an older one.
In addition we might want to also add an optional field for specifying this manually.
This could either just be an integer field, when if present is used for sorting. Any value > 0 would mean that the feature would get precedence to those that don't have the field specified. The inverse with < 0.
Or this could be a reference to another entity (through the $eid feature) with a specification of "above" or "below".