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/messageTray.js

@@ -20,9 +20,6 @@ const Params = imports.misc.params;
 const Tweener = imports.ui.tweener;
 const Util = imports.misc.util;
 
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-
 const ANIMATION_TIME = 0.2;
 const NOTIFICATION_TIMEOUT = 4;
 const SUMMARY_TIMEOUT = 1;