Rename ADDON_ICON_SIZES and only generate the ones we need (bug 885329)

(we should probably deprecate 32x32 and 48x48 as well, but that's
 for another bug)
Merge pull request #2223 from diox/stop-generating-useless-icon-sizes

Rename ADDON_ICON_SIZES and only generate the ones we need (bug 885329)