Moved action handler related code into a separate file

Since the action handlers are getting quite numerous and make the process-module.lisp file seem a bit cluttered, they are moved to a designated file.
Replaced `pr2-manip-pm' by the full name `pr2-manipulation-process-module'

The nickname inside the .asd file is left there for compatibility reasons for now.
Added the `missing' predicate to list objects missing in a list
Added the `flatten' predicate
Added predicate to find available arms w.r.t. objects in hand
Added new, cleaner predicates for grasp decisions
Removed the `*handle-...' prefix from grasp- and pregrasp poses

The given poses are valid for all kinds of objects, not just handles.
Removed obsolete predicates