Renaming "properties_object_constraint.py" to "properties_constraint.py"

The code in this file is NOT restricted to use in object context only. Renaming
it makes it easier to find this file (taking in account name truncations).


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55183 954f8c5b-7b00-dc11-b283-0030488c597c