Add bedrock floor in slab mode

[Loobinex]

Moved here from dkfans/keeperfx#668

There are currently no dungeon keeper maps that I know of that use 'unclaimable' slabs. My guess is that this is because mapmakers don't know they have that option. It would be very nice for mapmakers to have access to a terrain type that can be crossed but not claimed.

I propose to add a 'Bedrock floor' slab to the ADiKtEd editor: A ground-level slab that has a rock-texture that cannot be claimed or dropped onto, but can be crossed by creatures no problem.

Here is a demo level that shows the slab in action:
demolevel.7z

It is currently possible in Adikted to make this slab, but it is very well hidden and a lot of work when placing a lot of slabs.

Current way to create a single slab (!) of bedrock floor:

Open ADiKtEd, start editing map
In 'Slab mode', press Ctrl+M to manually select a slab
Select 50:Unknown slab to place it.
Press C to enter Column mode
Press M to change the column
Select 36: Ground level rock