Allow user to select which card displays as the face of a deck

Both desktop and mobile have logic for determining which card should be shown on a deck's "box", when rendering decks in image view:

https://github.com/Card-Forge/forge/blob/349129f88f00c611e6b75b70aa9fce6065fa2861/forge-gui-desktop/src/main/java/forge/itemmanager/views/ImageView.java#L1176-L1179

https://github.com/Card-Forge/forge/blob/349129f88f00c611e6b75b70aa9fce6065fa2861/forge-gui-mobile/src/forge/itemmanager/views/ImageView.java#L1141-L1146

It should be possible for a user to be able to override this manually.

A good first step would be consolidating those two bits of logic for picking a face card within `DeckProxy`. The chosen face card could easily be stored as a field in `Deck`, serialized as one of the parameters at the top of a deck file, and used to override the automatically selected face. Then we'd just need a way for the user to choose or clear the face card in each deck editor. The dropdown that appears when right-clicking a card would make a sensible place for this.

Additional considerations - if cards not in the deck should be usable as a deck's face; if there should be an additional option to set the back face of a DFC as the deck's face; if yes, will that extend to weirder alt-face types like specialize and meld?