Prevent overwriting quest's description and game's objective.

[MarcCote]

At the moment, manually providing the description of a quest doesn't work as intended. It gets overwritten by the text generation process when building/compiling the game. The same thing is happening with the game's objective.

Code to reproduce the error:

import textworld

M = textworld.GameMaker()
r1 = M.new_room()
M.set_player(r1)

key = M.new(type="k", name="key", desc="This is a skeleton key.")
r1.add(key)

quest = M.set_quest_from_commands(["take key"])
quest.desc = "Find a valuable object."

game = M.build()
assert key.infos.desc == "This is a skeleton key."
assert quest.desc == "Find a valuable object."