Improve icon selection in paragraphs type form.

Re-rolled patch against latest 8.x-1.x-dev branch.

The patch from #17 misses a lot of code changes from #10, therefore hiding it. The MR diff contains all the code changes from #10 and also fixes the deprecated usage of file_create_url(). You should use the MR and its diff instead.

I rebased branch 2854585-improve-icon-selection with 8.x-1.x and improved the calls to file_create_url() resp. the file_url_generator service.

Just found a small bug with this patch: The following element is added to ParagraphsTypeForm::form() method's form array by this patch:

<?php
$form['static_icon_file']['description'] = [
  '#type' => 'item',
  '#description' => $description,
];

Unfortunately an element with '#type' => 'item' is treated as an input value and thus interferes with the form's actual description form field value.

So my changes add '#input' => FALSE to the new $form['static_icon_file']['description'] element to NOT treat this element as a form value anymore.

I'd really like a way to set icons that don't have to be uploaded and stored in the files directory. It would be ideal to be able to just specify a path to an icon, so it could be included with the theme. I would also like the option of using something like fontawesome to provide icons, so being able to just put in an HTML tag that displays an icon would be great. Then it's on me to make sure that the icon font is included with the admin theme for the sites I build, which I already do and use in other areas.

@teknocat Hi. Did you had a change to test the merge request we've made?
After applying the changes you are able to implement hook_paragraphs_type_static_icon_uri_alter() (in a module or theme) where you can set the icon path to wherever you want. The file could be saved in a module, in the a theme, on an external storage, you name it ...

To use icons provided by fontawesome, you'll nee to override/decorate the ParagraphsTypeListBuilder class and change the way, $row['icon_file']['data'] is build.
Example (not tested):

<?php
      $row['icon_file']['data'] = [
        '#theme' => 'html_tag',
        '#tag' => 'span',
        '#attributes' => [
          'class' => ['fa-' . $entity->id()],
        ]
      ];
?>

This will create a <span> tag using the paragraph type ID as fontawesome class.

@stborchert ah ok! Sorry I don't think I fully understood the possibilities with that merge request.

I'll try that out.