Link plugin ruins undo system

[Sa'ar Zac Elias]

Steps to reproduce

• Open the replace by code sample
• Select all content and remove it. An undo snapshot is made.
• Add any element that is not a link with the toolbar [e.g. anchor, image etc.]. Another undo snapshot is made.
• Press undo once, so there is no content again.
• Add a new link.

Notice that the 'redo' button is enabled for no reason. [clicking on it, btw, doesn't do anything].

• Press undo twice.

Notice that the content is back to the default, so actually the two latest snapshots are skipped.
Take into consideration that no snapshot is made when creating link (as i reported earlier in #5385).

[Garry Yao]

Depends on #5385.

[Frederico Caldeira Knabben]

I confirm it's working properly now. Changelog entry added with [5719].