Changed _drupalgap_form_submit() to build two variables to hold onto the validation and submission mechanisms so that image fields could be preprocessed asynchronously. Moved the form.action and drupalgap_goto() mechanism into the submission processor.
Changed drupalgap_entity_form_submit() to set the form.action and let the DG form submit processor handle the drupalgap_goto() call.
Added _image_field_form_process() and changed image_form_alter() to not attach custom validate or submit handlers, since the image is processed before validation and submission begin now via _image_field_form_process().