Bug 816298 - Part 1: Allow child elements to override -moz-user-select:none. r=ehsan

--HG--
extra : rebase_source : f4e08713be0691c25b22510977c978088655ec8b
Bug 816298 - Part 2: Replace references to -moz-user-select:-moz-none with -moz-user-select:none. r=ehsan r=enndeakin

--HG--
extra : rebase_source : 4763e08113fb49a282fc04af072f60b74046e2c4