Prevent save event instead of submit

[Piotr Jasiun]

Working on #10689 I realized that ​this part of code works magicly:

// #8031 If textarea had required attribute and editor is empty fire 'required' event and if
// it was cancelled, prevent submitting the form.
if ( editor._.required && !element.getValue() && editor.fire( 'required' ) === false ) {
	// When user press save button event (evt) is undefined (see save plugin).
	// This method works because it throws error so originalSubmit won't be called.
	// Also because of hack there is try-cache in save plugin so error won't be shown.
	evt.data.preventDefault();

It should be fixed using save event instead of submit.