Add stepper to document window (gdiff, 466ab7f)

Add placeholder stepper and text to the document window. This will later be used to step through changes within a file (option-click will allow you to cross file boundaries if you wish) while showing status text of the form, "Showing change %d of %d (%s)" (two numbers and a change-type string such as "insertion", "deletion" or "edit").

Signed-off-by: Greg Hurrell <greg@hurrell.net>