Fix this and other issues more globally by moving the logic that
formats the image's display name into the GimpImage object, and return
the properly formatted name, e.g. "Foo.xcf", or "[Foo] (imported)"
from gimp_image_get_display_name().

Also add gimp_image_get_display_path() which returns the full path
instead. Use the two functions for formatting the image title, and
apply various other fixes that make sure the UI always uses the same
string to identify the image.

Call gimp_object_name_changed() whenever the save/export status
changes, so the image's cached display name and path get cleared.

(cherry picked from commit 220b2867)