Allow an asset pack to assign a stage to a character

[Wolfy76700]

For custom graphics, TSH should allow for assigning "home stages" to each character in the game asset config, as long as the stage is defined in the game asset config as well. For instance, in Rivals of Aether, Zetterburn’s stage would be Fire Capital, so you could have a layout which loads the Fire Capital stage background asset when Zetterburn is selected, independenly from stage strikes.