add related card pictures (dfc)

[tooomm]

I highly like to see the back sites of Double-Faced-Cards in the preview window!

Otherwise you have no way to know about the back site in draft or during building of limited pools.
So this information is very critical, especially for new sets or cubes with older or less common cards.

It could look like this:

• normal DFC (transform, flip)
  
• special DFC (melt)
  
  Where the both halves actually show the combined melted card in normal card size.

Back sides should be displayed a bit transparent for all of them.

allsets.json from mtgjson holds useful information about the relevant cards.
In the "layout" key they are tagged with "meld" or "double faced" and have the names of the other sites/cards, in that object too.