[MAJOR] Remove .ajax and allow async function for .setChoices

[tinovyatkin]

Removes .ajax method and introduces similar functionality by extending .setChoices

Description

Remove .ajax method due to bad name (it's not immediately clear what it does, and soon probably nobody will understand acronym, as I didn't remembered that AJAX stands for Asynchronous Javascript And XML 😳) and outdated functionality concept.

Introduces abiltiy to pass a callback or async function as first parameter to .setChoices to provide similar functionality for today' Promise world.

Motivation and Context

Almost all modern day fetching APIs are Promises based (all your examples except one are), while we are providing callback 😏 way to do that.

This also removes fetchFromObject helper function and delegates to consumer to do complete data transformation (fetchFromObject handles just one use case, without arrays, etc, and complete solution will require to introduce a huge dependency - so, it's better to let a consumer do whole transformation).

NOTE: This PR is not adding direct dependency on Promise implementation in Browser, so, not adding Promise to required polyfills.

How Has This Been Tested?

Refactored existing test and introduced new one.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[tinovyatkin]

Just waiting for #700 to merge and rebase to remove that's code from here.

[tinovyatkin]

This also closes #604

[tinovyatkin]

Also closes #626

[jshjohnson]

This is much cleaner - thanks!