[DX] FAPI: Dropbuttons: Also support <input> (AJAX calls) for the '#links' items in '#type' => 'operations'

[klonos]

This would help certain popular contrib projects, like Paragraphs, to standardize their UI instead of having to add cruft to implement their own solutions.

From comments in backdrop-contrib/paragraphs#62

@irinaz

@laryn , let me know where is good start to move forward to implement these features

@laryn

@irinaz If you look where those buttons are added, you can start a branch/PR for this ticket and test out converting them to Backdrop's "dropbutton" format. Look at $element['inner_actions'] inside paragraphs_field_widget_form_build().

@herbdool

The hitch is that the dropbutton library was built to work with the operations form field type (which creates a list of <a>) rather than <input> with submit handlers and ajax callbacks. I'm not sure if the latter can easily be converted to <a>.

There's a fuller answer here: https://stackoverflow.com/questions/18026951/drupal-ajax-call-from-click-of-a-link-in-form