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/accessibility.js

@@ -14,9 +14,6 @@ const PanelMenu = imports.ui.panelMenu;
 const PopupMenu = imports.ui.popupMenu;
 const Util = imports.misc.util;
 
-const Gettext = imports.gettext.domain('gnome-shell');
-const _ = Gettext.gettext;
-
 const A11Y_SCHEMA = 'org.gnome.desktop.a11y.keyboard';
 const KEY_STICKY_KEYS_ENABLED = 'stickykeys-enable';
 const KEY_BOUNCE_KEYS_ENABLED = 'bouncekeys-enable';