Create further differences between micon type "image" and "font"

Problem/Motivation

Currently, the Micon Entity types "image" and "font" are used inside code as a class variable. Instead we should have the Micon entity as a parent class and two classes "MiconFont(Entity)" and "MiconImage(Entity)" which extend from "Micon" and implement their very own functions (Find out the differences between image and font type first through #3332223: Refactor selection.json modification).

This makes the code way cleaner, as we introduced micon "font" specific class variables (fontFamily) through #3325584: Make font-family adjustable in the UI.

We should also discuss, if this splitting should be also taken into the UI, or only used in code.

Steps to reproduce

Proposed resolution

Implement two classes "MiconFont(Entity)" and "MiconImage(Entity)" which extend from "Micon" and split the Micon functionality there.

Remaining tasks

User interface changes

API changes

Data model changes