Browser with Multiple Add Buttons

Problem/Motivation

In a larger effort to make Paragraphs UX better for end users and editors there is a need for a better browser but also allowing more inline insertion of a new paragraph. For example large articles with 10-20 paragraphs associated if a new paragraph needs to be added at delta 3 a user has to add to bottom of list and drag up sometimes awkwardly to the 3rd position.

Proposed resolution

Work has begun on adding "add" buttons between each existing paragraph that allows insertion of the paragraph between existing paragraphs in #2877927. This is dependent on some modal functionality being worked on within the core paragraphs module. A better solution might be building upon this to add the better UX functionality offered in Paragraphs Browser for powering this modal. On most cases sites that need insertion between Paragraphs also are going to have a larger number of paragraphs and benefit from the more user friendly UX of paragraphs browser.

Remaining tasks

• MVP - Initiate paragraphs browser modal from between paragraphs buttons - DONE
• MVP - When paragraph is added from browser insert into from - Not currently working
• Cleanup ajax response commands
• Consider future uses or extending

Steps to Reproduce

- Setup D8 site with patch from comment 4 in #2877927 added to paragraphs module.
- Apply patch from this issue to paragraphs_browser
- Edit form display for paragraphs browser and save
- See add buttons between paragraphs but upon adding see that the paragraphs do not add from either new add buttons or the existing Add Paragraph button provided by Paragraphs Browser module
- Change "add_mode" setting from modal back to "paragraphs_browser". Save form display again and reload. See add buttons no longer show but the Add Paragraph default from Paragraphs browser showing.

    $elements['add_mode'] = array(
      '#type' => 'hidden',
      '#value' => 'modal',
    );