change runscreen vars

[stefanfaistenauer]

replaced payload and credential json with list of items available to be set.
error handling on missing vars.
support of json payload through json parsing on brackets

[stefanfaistenauer]

also new json editor on edit and schema create

[nimarb]

• in the API Config window, the entire window now scrolls, not just the JSON editor (worked before)
  
• json editor can't scroll at all in the create stepper
  
• variables in the Run window missing in existing integrations