New Slab: Dense gold

[Loobinex]

In Dungeon Keeper there's currently 2 valuable slabs:

1: Gold, which holds 1024 gold which is mined quickly and when the gold is gone, it crumbles and opens up the map.
2: Gems, which hold infinite gold, but can be mined from slowly and acts as a permanent barrier.

A third option would be a good addition:

3: Dense (better name pending): Like gems can be for a long time, dig out slowly, but contains a limited amount of gold and will crumble after a long duration.

[PieterVdc]

discussion of visual parts and wich other variants here
https://discord.com/channels/480505152806191114/1334082860914708491