environment: put gettext stuff into global environment

Rather than defining _() as a local function in every module, put it
into the global environment (along with C_() and ngettext()).

https://bugzilla.gnome.org/show_bug.cgi?id=649203

js/ui/docDisplay.js

@@ -1,8 +1,5 @@
 /* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
 
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-
 const DocInfo = imports.misc.docInfo;
 const Params = imports.misc.params;
 const Search = imports.ui.search;