Menu slider colors

[Rachael]

This is a quicky I made mostly for my own self. I figured I would share it, though.

All it does is it changes the menu slider colors in the recent dev builds to be more thematic to their respective games, rather than the static orange it used to be.

Reason I did not commit this directly to GZDoom is because this, by itself, could probably disrupt the theme older mods are going for - I don't know, really, it doesn't really matter, it's just a minor cosmetic change, and it's easy enough to make mods like this to suit our own tastes, eh?

These are the games it will change, and to what:

Chex - Green
Doom - Red
Heretic - Green
Hexen - Red
Harmony - Ice Blue
Strife - Orange