Improved MultiStep selection display (DropZone Widget)

Problem/Motivation

In order to improve user experience with reduced action steps in Entity Browser, this ticket is created: #2822009: Improved MultiStep selection display. In that ticket adding/removing elements in multistep selection display is done over ajax requests, where overall user experience is way better.

We want to add support for usage of provided functionality of multistep selection display with upload widget. And one of upload widgets for entity browser is dropzonejs submodule -> dropzonejs_eb_widget.

Proposed resolution

Possible solution to trigger automatic selecting of entities with dropzonejs widget is to trigger submit process of widget and after it's successful to return list of added entities. That list can be used to trigger selection of entities in multistep selection display.

In this case process is a bit longer then in case of view widget, since files has to be uploaded and entities created, before we can trigger selection in multistep selection display.

These are steps:

1. drop files in dropzone
2. they are automatically uploaded and validated
3. after validation and upload is finished, clear all rejected files, so that at end we have files that can be used to create entities
4. over ajax request, automatically submit form and that will create entities and list of created entities will be returned
5. returned list will be used in javascript to trigger selection in multistep display (and this part is similar to view selection widget)

Remaining tasks

• Same functionality should be provided for default upload widget

User interface changes

In user interface there is change that "Select entities" button is not displayed if widget is using "auto_select" functionality. Because that flag means that widget it self will handle selection and there is no need for default submit action to be trigger by user.

*Edit* Usability example (with keeping list of rejected files)