gundam: Make StaticBind variables be available in Squirrel initializer.

These variables are not only customizable in initializer script
(ReZEL.nut in this case), but also modify on the fly (while the game is
running).  And we do not need to register a variable twice.

Currently, StaticBind variables are not mandatory in initializer scripts.
gundam: Add key bind of some commands for ReZEL.

The added commands are: reload, get cover, toggle stabilizer and
transform.  Get cover command is not testable for the moment.

Old binding codes are removed from gundam/demo.nut because they should
be called for all ReZEL instances, not only for demo.nut case.