Promise support

[jacekbogdanski]

Type of report

Feature request

Provide description of the new feature

It seems to be a good time to provide Promise support for CKEditor4. It's already required and somehow implemented in our testing environment (see #2752) although it would be nice to have dedicated CKEDITOR.tools.promise as a wrapper around native Promise object which will also handle polyfilling for IE. We should discuss polyfill library options - it doesn't make sense to spend time reinventing the wheel again.

Some requirements / things to discuss :

1. Use native Promise everytime when it's possible.
2. Load polyfill library asynchronously only for the browsers with no Promise support when needed.
3. Choose polyfill library with the same API as Promise.
4. Hide Promise under CKEDITOR.tools.promise namespace so we won't overwrite native objects.