ENH Migrate image selection logic from asset-admin

[GuySartorelli]

A TinyMCE 6 compliant version of https://github.com/silverstripe/silverstripe-behat-extension/pull/248

Migrates logic for selecting an image in the TinyMCE WYSIWYG from silverstripe/asset-admin as it's now needed in silverstripe/cms as well.

Make sure you check the installer CI run linked in the issue to see behat passing happily

Required for

• https://github.com/silverstripe/silverstripe-asset-admin/pull/1398
• https://github.com/silverstripe/silverstripe-cms/pull/2884

Issue

• https://github.com/silverstripe/silverstripe-admin/issues/1548