Make “dirty fields” work for elements without drafts

[bencroker]

As of Craft 4, elements keep track of changes to their attributes and fields using the $_dirtyAttributes and $_dirtyFields properties when the element is saved directly, and using the $_modifiedAttributes and $_modifiedFields properties when the element is saved via a draft.

When saving an element that does not support drafts such a User, however, $_dirtyFields stores all custom fields regardless of whether any field values were actually changed. Additionally, the non-existent field changes are registered in the changedfields database table.

Steps to replicate:

1. Add a custom field to the user field layout.
2. Visit the user edit page in the CP.
3. Don’t change the custom fields value.
4. Look in the changedfields database table and see that a row is added for the user and field (or the dateUpdated column is updated if the row already existed).

It would be great if $_dirtyFields would track only fields that have actually changed, much like $_dirtyAttributes only tracks changed attributes.

This would additionally make the content in the changedfields database table more accurate, helping to pave the way for a reliable element history feature.