Add Primary Key Icon for Entities and Aggregates

[hevselavierlines]

This is a functionality for both the GUI element and the property table to choose which properties should be the primary keys. The primary keys should not be deletable and has to be unique. In most cases the first property called ID is this unique property.

The key is also automatically generated from the database.

[hevselavierlines]

Solved the issue:

• Add png icons for add and remove button
• Add icon support to show the primary key
• Add a property table entry for the primary key
• Primary key functionality only for Entities and Aggregates.
• Value Objects are in an own class and should not support Primary Keys as they do not use an unique id property.