Bug 1000513 - Allow menuitems in menupopups to be children of a menugroup for horizontal arrangement. r=enn

--HG--
extra : rebase_source : 9e6f99b95e48f26aae2c3f7b8ee436781451a299
Bug 1000513 - Combined navigation items in the context menu. r=Unfocused,dao

--HG--
extra : rebase_source : 90f7c49cf5b4917527bc310d74f5f8327191ca22
Bug 1000513 - Tests for menugroup keyboard navigation. r=Unfocused

--HG--
extra : rebase_source : 925b6991f3444ff197b166f1e4234f1f19fe321d