Saving SBCL images with cl-gtk2

SBCL's image saving now works with cl-gtk2. It took fixing several issues:

• Stop the Gtk+ event loop when saving an image
• Initialize Gtk+ when loading an image
• Re-register Lisp-implemented GObject classes with Gtk+ upon loading an image

Using cl-launch or by just invoking sb-ext:save-lisp-and-die, it is now possible to create a Lisp image with an application that would start much faster than even loading from fasl files.