CKE 'split' plugin triggers misleading Undo UI

## To Reproduce

Step by step instructions to reproduce the behavior:

> 1. Enable split in a rich text singleton
> 2. Add text
> 3. Use Split

## Expected behavior
Split's deletion of a rich text widget should bypass the Undo mechanism

## Details

**Version of Node.js:**
v12.16.1

**Server Operating System:**
macOS 10.15.4

**Screenshots**
http://g.recordit.co/lpbJMeH9XK.gif

Reproduce the issue by enabling split in a rich text singleton, adding text, and using Split. Start by tracing the CKE split plugin's deletion behavior and the Undo UI; done means deleting the rich text widget bypasses Undo as described.

Written by the indexing model from the issue text.