Decouple form specific effects from the actual form.

Most hardcoded form == FORM_* checks all over the place have been replaced.
Now, forms change certain avatar properties, which are in turn checked
in the places that previously had these explicit form checks.
This allows specifying functionality via scripting, for additional forms
with new effects or a new combination of existing effects.

OT_HURT now deals damage of type DT_WALLHURT,
and by setting avatar->setDamageTarget(DT_WALLHURT, false) collision
with walls can be made harmless (exactly as nature form used to be).

Add related Lua functions:
+ setWorldType() - Change WT without actually switching to spirit form
+ setCanActivate()
+ avatar_setCanBurst()
+ avatar_canBurst()
+ avatar_setCanLockToWall()
+ avatar_canLockToWall()
+ avatar_setCanSwimAgainstCurrents()
+ avatar_canSwimAgainstCurrents()
+ getSceneColor()
+ DT_WALLHURT
Forms decoupling #2.

Added Lua functions:
+ avatar_setCanCollideWithShots()
+ avatar_canCollideWithShots()
+ avatar_setCollisionAvoidanceData()