Add NOGRAVITY to every Raven actor with FLOATBOB

- Fixed: Heretic and Hexen completely skip all normal Z processing on
  mobjs with FLOATBOB set. To emulate that, we need to add NOGRAVITY to
  everything with that flag set.
Change tspeed += 2 to *= 2

- Fixed: The initial keyboard turn rate used the running initial rate whether
  +speed was down or not.
Don't display the cost in dialogues twice

- Fixed: Commit 75dd5503, which was to enable the use of LANGUAGE
  substitutions for conversation items with a cost attached, neglected to
  remove the original code that attached the cost to the end of the item,
  so the cost got added twice.