link to output, emission viewer and "poll" changes

1. In "Link to Output Node" and in "Emission Viewer" changed behavior
when sockets are hidden.
Even when sockets were hidden - links were made from those sockets. Now
it's corrected.
2. Restricted add-on to work only in Cycles Shaders and in compositing.
Addon is not active for Blender Render material nodes and for texture
nodes.