Fix #37338: Add objects from addon work in a Linked Scene

Added a poll function to add object helper which checks
whether scene is linked or not.

All the primitives which are delivered from this helper
will work properly for linked scenes. If there're still
primitives which are not delivered from this class, well
nothing i can do now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61183 954f8c5b-7b00-dc11-b283-0030488c597c