Add support for HTML5 "required" attribute in form input elements

[Urs Wolfer]

It would be nice to have a checkbox for setting the required attribute for form elements.

[Jakub Ś]

We have done it for base textarea that classic CKEditor is using (#8031) so it would be nice to add such field to input form elements CKEditor handles.

[Urs Wolfer]

Just to make it more clear: I'm talking about a checkbox in the forms plugin for each input field to enable / disable the required flag.

[Urs Wolfer]

​https://github.com/ckeditor/ckeditor-dev/pull/100

[Matti Järvinen]

Nice... optional placeholder while you're at it?

[Urs Wolfer]

Replying to matti:

Nice... optional placeholder while you're at it?

I'd like to get this merged first. Please open a new feature request for placeholder tests (if there does not exist one yet).

[Olek Nowodziński]

I extracted the code from ​https://github.com/ckeditor/ckeditor-dev/pull/100 to ​branch:t/11583 and thoroughly reviewed it. Changes include:

• Tests separated from dev code.
• Improved code style.
• Some new and improved tests.
• A different accessKey for "required" field. I decided that it got to correspond with our naming pattern, so it's "R". I also checked whether it collides with browser keystrokes but it seems to be fine.

[Piotrek Koszuliński]

I pushed ​branch:t/11583 rebased on major (since it's a feature).

I agree with ​this comment that in this case 'Q' may be a choice.

I made just a brief check and I'm ok with the changes.

[Olek Nowodziński]

Fixed in ​git:a067471.