Edit > Undo remains disabled after paste or cut on newly opened sketch

Using Arduino IDE 1.8.14 Hourly Build 2021/30/09 09:33, Windows 10 64 bit

1. **File > New** - this will cause the **Edit > Undo** menu item to be disabled because there is no edit history.
2. Copy some text to the clipboard.
3. Paste the text to the sketch - **Edit > Undo** remains disabled even though a **Ctrl+z** will undo the paste.

This issue occurs with Arduino IDE 1.8.2 and newer, it does not occur with 1.8.1 and older.

This may be related to the `DocumentChanged` event issue mentioned in https://github.com/arduino/Arduino/pull/6022#issue-210557822.

Originally reported at http://forum.arduino.cc/index.php?topic=443558.msg3289844#msg3289844

Reproduce the paste and cut cases in a newly opened sketch using the Edit > Undo menu and Ctrl+Z. Start by reviewing the DocumentChanged event issue referenced in pull request #6022 and the undo action's enabled state; done means Edit > Undo becomes enabled after paste or cut and performs the same undo as Ctrl+Z.

Written by the indexing model from the issue text.