fixed a bug in poly redux's vgroup weight merging (was reducing the weight each collapse by about half)

fixed some other UI logic in the python menu script
Added an option to use a vertex group for a reduction weight map to force reducing some areas more then others.

Mesh epydocs activeGroups can be None as well as string.

source/blender/python/api2_2x/doc/Mesh.py

@@ -693,7 +693,7 @@ class Mesh:
   @type activeFace: int
   @ivar activeGroup: The mesh's active vertex group.  The mesh must be
     linked to an object (read the comment in L{addVertGroup} for more info).
-  @type activeGroup: string
+  @type activeGroup: string or None
   """
 
   def getFromObject(name,cage=0):