Add a backgroundless option for stages in the settings#18

With the update to UE 5.8, a lot of old mods stopped working, which includes now the potato stages mod. This helps a lot of weaker PCs be able to play the game or not worry about stutters while streaming.

A native background elements removal option would help the processing of the game to run better for these players, and can instead simply have sky gradients like the mod had for stages like Godai Delta.

Sell us the training room skins for each stage!