allow a hidden pref ("imageblocker.enabled") to hide image blocking context menu UI.
note, this fix is 1.7 only, and will not be landing on the trunk.
r=mconner,sr=dveditz,a=chofmann
git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_7_BRANCH@156864 18797224-902f-48f8-a5cc-f745e15eee43