Fixed Main Menu

Tried to re-use the main menu's lateral arrows when rebuilding the pattern-selector in 2P versus. In saving the arrow nodes into individual scenes, I broke some unknown connection that made the arrows not adjust their positions with menu items. 

The solution I went with was to delegate managing arrow positions to the arrows themselves whenever they are visible: they check their position on every frame they're on-screen, but stop once they notice they're hidden.

Completely forgot that the pattern selector was broken, tho.