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/endSessionDialog.js

@@ -22,9 +22,6 @@ const DBus = imports.dbus;
 const Lang = imports.lang;
 const Signals = imports.signals;
 
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-
 const Clutter = imports.gi.Clutter;
 const Gdm = imports.gi.Gdm;
 const GLib = imports.gi.GLib;