lookingGlass: Include St in default imports instead of Gtk

Until commit 467b7c1bcacebdc, the import used to leak into the
eval() environment, but not anymore. Add it back (and remove
Gtk, as it's not *that* useful).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/398

js/ui/lookingGlass.js

@@ -17,7 +17,7 @@ const JsParse = imports.misc.jsParse;
 const CHEVRON = '>>> ';
 
 /* Imports...feel free to add here as needed */
-var commandHeader = 'const { Clutter, Gio, GLib, GObject, Gtk, Meta, Shell } = imports.gi; ' +
+var commandHeader = 'const { Clutter, Gio, GLib, GObject, Meta, Shell, St } = imports.gi; ' +
                     'const Main = imports.ui.main; ' +
                     'const Mainloop = imports.mainloop; ' +
                     'const Tweener = imports.ui.tweener; ' +