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.