It should not be a form's responsibility how its submit button should look..

...in case it's a standard button. No need to be DRY here.

refs #5525
Check whether a form's data was sent instead of whether it's complete

It is not sufficient to just check whether all required elements are being
submitted. We definetely need to check whether the submit button was
pressed. But doing this and providing a standard button with a static name
simultaneously will produce conflicts if forms are using the same action
urls. To fix this, we'll add an additional form-identification check by using
a form's name or class.

refs #5525
It is a form's responsibility to process the request it was submitted with

How forms are being validated should not be a task that is part of the
controller action, but should be handled by the form base class as it is
most of the time the same procedure. Now a controller action just sets
up the form to use, calls handleRequest() and acts upon its return value.
(e.g. calling handleRequest() from another form or any redirection)

refs #5525