Provide a minimal custom elements + properties panel example

[nikku]

Tasks

• Showcase custom-elements + properties integration
• Showcase custom element editing via properties panel

Originally posted by @nikku in #66 (comment)

[philippfromme]

+1 This would help a lot of people.

[billbarni]

I am having trouble rendering a custom component.
When I run the properties-panel example project, the React component works, but when building and adding the plugin to the Camunda Modeler, the component does not get loaded, the properties panel is empty without any errors in the console.