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/status/network.js

@@ -18,9 +18,6 @@ const MessageTray = imports.ui.messageTray;
 const ModemManager = imports.misc.modemManager;
 const Util = imports.misc.util;
 
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-
 const NMConnectionCategory = {
     INVALID: 'invalid',
     WIRED: 'wired',