correct addon tutorial section on keymap register/unregister and add label next to image color-depth option.
having buttons labelled [8, 16] on their own is not very meaningful.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53981 954f8c5b-7b00-dc11-b283-0030488c597c