Clean up build restrictions categories.

[sanderd17]

Currently, the build restrictions categories are used in two ways.

There's the dock case, where the rotation (and additional build restrictions about water depth) depend completely on that one tag. And there's the Civil Centre case, where the distance to each other is determined by the tag. All the rest are regular buildings, and don't even use the tag.

This also means it can collide. When the brit_crannog is to be used as a replacement CC, but has the same placement criteria as a dock, you cannot achieve this.

IMO (but this needs discussion first), the categories should only be used for special terrain-dependent changes (s.a. rotation and ground elevation in the dock case). While the MinDistance check should happen on cmpIdentity classes. This would allow the crannog to get the CivilCentre class, and a minimum distance from all entities with a CivilCentre class. While it also keeps the Dock category, so it's rotated as a dock.

As a concequence, we shouldn't have all those different categories present. For what we're implementing currently, a "Dock" and "Regular" category are enough. If there come new special cases, other categories can be added.

[sanderd17]

In 13852:

change build distance restrictions to work with classes. Fixes #2102.

[sanderd17]

In 13853:

remove warn messages. Now really fixes #2102