Add optional class field to add further classes on the icon element

Problem/Motivation

It's often required to set utility classes on the icon, for the size, the color, .. and so on.

Steps to reproduce

Proposed resolution

Add a simple class textfield to enter further classes (separated by space).
Bonus: It might be configurable on the form widget, if the class field should show up or not.

Remaining tasks

User interface changes

API changes

Data model changes