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/telepathyClient.js

@@ -9,9 +9,6 @@ const Signals = imports.signals;
 const St = imports.gi.St;
 const Tpl = imports.gi.TelepathyLogger;
 const Tp = imports.gi.TelepathyGLib;
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-const C_ = Gettext.pgettext;
 
 const History = imports.misc.history;
 const Main = imports.ui.main;