Possibility to add an icon to any form type

[vitek-rostislav]

Describe the expected outcome of your requested feature

In administration, there are a few form types that allow adding an icon, icon title, or symbol after input. This is ensured by appendix_block in theme.html.twig. I would like to have the possibility in all the form types (e.g. it would be very handy in DisplayOnlyType.) I have to implement it myself on my project now - extending displayOnlyFields.html.twig template, add {{- block('appendix_block') -}} there, and then define the icon in my form type extension by adding

'attr' => [
                    'readonly' => 'readonly',
                    'class' => '',
                    'icon' => true,
                    'iconTitle' => 'whatever',
                ]

(I have to re-define the default values of the first two attributes - readonly and class - to make it work).

[vitek-rostislav]

Today, I figured, I do not need to re-define the class atribute default value in teh form type - I can add the default value in the theme:

+ <div class="form-line {{ attr.class|default('') }}">
- <div class="form-line {{ attr.class }}">