Create constant for menu cursor color

[aplumafreak500]

Multiple functions call either CreateBlendedOutlineCursor or MenuCursor_Create814A5C0 with its fourth argument set to 0x2D9F. This gets written to gUnknown_0203A360, which the sprite template for the cursor itself points to as a palette.

By making this a constant, it should make it easier to change this color, given that it is set in a lot of different places.