Fix Show Brush button being missing from the UI for paint modes, the feature was
already implemented, it's in the Appearance panel now. Also added that panel to
the image editor now since it's relevant there too.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56449 954f8c5b-7b00-dc11-b283-0030488c597c