Add layout icons to the toolbar in CKEditor 5

Problem/Motivation

We had a hard time making the layout icons display on the toolbar buttons. So we used text buttons based on the layout labels instead. It would be nice to make the layout icons work as a visual representation of the layout is more clear the layout label.

Steps to reproduce

TBD

Proposed resolution

TBD

Remaining tasks

Find a solution to make the layout icons work.

User interface changes

The toolbar shows icon buttons instead of text buttons when inserting a layout.

API changes

N/A

Data model changes

N/A