setup: Add key_mission to the controls group.

Otherwise, the same key can be bound to "Show mission" and one of the other
"More controls..." at the same time.
heretic, hexen: Fix mouse (and joystick) weapon cycling.

Implement this the same way it is implemented for doom and strife.
Merge pull request #318 from svdijk/bugfix

Heretic/Hexen/Strife bugfixes from Sander van Dijk.