Create a plugin-template

[Hallbergs]

I've noticed that some hajk-developers are creating Class-based components instead of functional components still. I cannot really find any reason as for why, but one reason could be that the Dummy-plugin is still written as a Class-based component.

I suggest that we create a new plugin-template (as a functional component) to guide the developers in the right direction.

We should also rewrite the Dummy-plugin to a functional component, but that's a separate issue.