[Backport] Move buttons definition to separate file

[RahulKachhadia]

Original Pull Request

#15194
When trying to add new buttons with new actions (besides reset, save and saveAndContinue) to a UiComponent form in the backend, it is needed to extend upon at least 2 files: Magento_Ui/js/form/adapter and Magento_Ui/js/form/form. The second file is easily extendable using mixins, but the first one is not: Mainly because the buttons definition is based on a local variable. This PR fixes this by moving the buttons definition in a separate file.

Description

This PR adds a new file buttons.js that is called for in adapter so that someone could create a mixin like follows:

RequireJS configuration:

var config = {
    config: {
        mixins: {
            'Magento_Ui/js/form/form': {
                'Foo_Bar/js/form/form-mixin': true
            }
        }
    }
};

And in Foo_Bar/js/form/form-mixin:

define([
    'underscore'
], function (_) {
    'use strict';

    var mixin = {
        'foobar': '#foobar'
    };

    return function (target) {
        return _.extend(target, mixin);
    };
});

Unfortunately, this PR does not deal with how to use foobar in the end, which is hugely complex.

Manual testing scenarios

1. Apply the PR
2. Confirm that form buttons like Save and Save And Continue are still working in the backend

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @rahul-kachhadiya. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.