#178. Renamed drupalgap_entity_edit_form_cancel_button() to drupalgap_form_cancel_button(). Added drupalgap_form_element_get_module_name(). Changed _drupalgap_form_render_element() to choose a module to look to for the hook_field_widget_form() implementation on field elements and flat elements, migrated element preprocess modifications to their corresponding hook_field_widget_form() implementation, merged the element item values into theme's variables before sending to theme(), and set a default value onto theme's variables for form elements. Changed theme_radios() and theme_select() to ignore the attributes placed inside options. Changed drupalgap_field_info_instances_add_to_form() to change null default values to an empty string. Added options_field_widget_form() and image_field_widget_form(). Added a default cancel button to system settings forms.