[Therion] Map previews not preserved when indirectly referenced

[Bruce Mutton]

Thanks for the explanation and the offer Stacho

>> Is that not what the m123 example is doing?
>No, because there the preview is just caused by another offset, not
>"preview" option in the map.

The above was the bit that clarified why they were different. 

>>Probably best solution would be if preview inheritance could be turned
>>on/off by some switch. Would it solve your problems?
>OK, I will put it on the TODO list. May be it will not be too
complicated...

If it's related and not too hard (and not already completed)- allowing
sketches to be offset as well would be nice.

Thanks
Bruce