PROP_STORAGE_TYPE was only notified if it was changing *to* EMPTY, in
gtk_image_clear_internal(). We did not notify when it changes *from*
EMPTY to something non-empty. We should as not doing so is confusing,
e.g. if a user wants to bind :storage-type to :visible if non-empty,
which I just did! So, in functions that apply an ImageType, now notify.

Also do so in gtk_image_set_from_definition, declared in imageprivate.h,
even though none of the function there are currently used anywhere.
(Should they be removed?)