Add support for custom activity icons in v3

[truthz03]

Is it possible to simply add icons to custom activities? I searched the source code and found out that the ActivityIconRegistry is used to add icons but the ActivityIconRegistry is not exported therefore I can't import it inside my code. Even if I can import the ActivityIconRegistry I'm not sure how to define the icon itself within my Angular application.

It would be really nice if I could set an IconProvider within the Activity attribute inside the backend. This provider should be a class (same like the OptionsProvider from the Input attribute) with a specific interface which returns a svg icon as byte[] or string. With this mechanism I don't have to add code to the frontend and can add an icon directly to the activity in the backend.

The DefaultActivityTemplate than simply has to check if the activity itself has an icon received from the backend and if not it should search inside the ActivityIconRegistry like it does for now.

[sfmskywalker]

I like it!