[DMN Designer] Documentation - Widget for links persisting links

Every DRGElement (BusinessKnowledgeModel, Decision, DecisionService, InputData, KnowledgeSource) must have into its properties panel a widget for adding links.

The widget must be just like this one:
https://redhat.invisionapp.com/share/JSQRRG2ZU8K#/screens/349635889
(A list which is populated by the popover).

Each item in this list is a DMNExternalLink*, this entity as only two string fields (name and URL).

• This is just a suggestion, the entity name can be improved.

Acceptance test

On both Firefox and Chrome

• Multiple links - scroll to view all
• Add multiple, delete some
• Open the link
• Switch between nodes
• Link name - long
• Link name - multibyte characters