Clean up launcher settings a bit

Three main changes in this PR:

1) The enums for the "easy settings" launcher options are now clearer and more explicit, and the enum assignments for a lot of graphics options are now fixed, as they were incorrect due to code rot.

2) Some of the command-line flags options have been recategorized: -window, -fullscreen_window, -nomotiondebris and -noscalevid are now Gameplay; and -fps is now HUD.

3) Since post-processing is required for nebula fading, it now defaults to on, and the flag has been changed to -no_post_process.

Author: Goober5000