Icinga\Web\Form::onSuccess() must not return TRUE

When using Icinga\Web\Form directly without creating a specific sub-class
and calling handleRequest() one must not get redirected.

refs #5525
Make the form in view scripts accessible using $form instead of $element

refs #5525
Access formdata internally using a public helper method

This way concrete form implementations can also simply fetch form
dependent request values without referring to the form's submission
method.

refs #5525
Replace Form::onFailure() with Form::onShow()

There is currently no usecase for onFailure() but for onShow() wherewith
the case is covered when a form needs to be prepared before showing it
to the user while the form was not submitted in any way.

refs #5525
A form is "submitted" when it was "sent" but no submit label was set

This enables us to use handleRequest() as is when no specific submit button
is required.

refs #5525