Support auto-completion of template values

[nikku]

Is your feature request related to a problem? Please describe

As a user I want to be able to get auto-completion hints as I edit a template property:

Describe the solution you'd like

• I'm able to define hints with my element templates
• Hints have clearly defined preview format (Support cards / complex data elements inside element templates #2)
• These hints will be displayed with the element for auto-completion (Add ability to auto complete fields bpmn-js-properties-panel#708) (as I type)

Describe alternatives you've considered

Keep element templates static.

Additional context

None.