No undo snap when inserting form fields

[Garry Yao]

The problem happened generally for all form fields.

Reproducing Procedures

1. Open the replace by class example page in FF;
2. Click inside bold 'sample text' word.
3. Open 'Checkbox' to insert a default checkbox field.
   • Actual Result: 'Undo' button state show that there's no undo snap.

[Garry Yao]

This patch is also supposed to other plugin which use insertElement to introduce DOM changes.

[Garry Yao]

Fixed with [3646].

[Artur Formella]

insertElement event adds undo snapshot.