New icon: CAMERA_STEREO

Adding only the .svg file, the PNG can make once the multiview branch is merged.
I wanted to commit this to avoid having conflicts in the .svg file if
someone works on new icons too.

The new icon is at X-23

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57498 954f8c5b-7b00-dc11-b283-0030488c597c