Packing and Unpacking siege weapons

[Pureon]

Immobile siege weapons, such as the Roman Ballista and Greek Bolt Shooter, should be transformed from their mobile defenseless form (packed) into their immobile attack form (unpacked).

Most of the immobile siege weapons will have unique packed mobile versions. See units/hellenes/siege_rock_packed.xml and units/hellenes/siege_rock.xml as an example of packed/unpacked siege units.

Whenever the immobile siege weapon is tasked to move to a new location, or ordered to attack an object outside of it's weapon range, the immobile unit will transform into the mobile packed form (such as a horse drawn cart). If the unit has been given an attack order and reaches its attacking range, it will unpack into its immobile attacking form. If a unit is asked to simply to move to a new location, it will move but not unpack when it reaches the location.

A button in the UI should toggle packed/unpacked. This will be used when setting up defenses etc.

Animations for the packing/unpacking transformations can be added at a later stage.