CompatForm: Insert duplicate submit button when more than one is present

[TAINCER]

Creates a new SubmitElement from the primary submit button of the form with cloned Attributes from the given FormSubmitElement. The new submit button is always at the top of the form.
The element has a fixed primary-submit-btn-duplicate class, which hides to button, but not the functionality when submitting the form.

[nilmerg]

What about overriding setSubmitButton() with this trait? It's used with child classes of ipl\Html\Form after all so it shouldn't pose a problem in the inheritance tree.

I'd like to just use the trait and be done with it. Calling a special method, in this case, is too tedious for me 😉

[TAINCER]

What about overriding setSubmitButton() with this trait? It's used with child classes of ipl\Html\Form after all so it shouldn't pose a problem in the inheritance tree.

I'd like to just use the trait and be done with it. Calling a special method, in this case, is too tedious for me 😉

if I would use the Trait in setSubmitButton(), the Prefix Button could be rendered multiple times. The commit for CompatForm was still missing, in there I've overwritten renderContent(), where I used the createSubmitButton() method.

[nilmerg]

I see. I forgot that this is only relevant if there are multiple submit buttons and then the set submit button isn't reliable. (even if available)

[nilmerg]

Please also add a another test case to CompatFormTest which verifies that an explicit call to setSubmitButton() duplicates said submit button, and not the first registered one.

[nilmerg]

LGTM!

[nilmerg]

Please squash your commits to the following:

• One for the trait + less file
• One for the composer.json
• One for the CompatFormTest
• One for the DuplicateSubmitButttonTest + TestCase

[lippserd]

PR title and description have to be adjusted as well.