Warn empty required fields in target

[igarashitm]

Right now UI does nothing for required fields on target object. it should

• have a indicator for the required fields (we can do it for source object as well)
• emit a warning if any of target required field doesn't have mapping
  • UX question: when? UI doesn't have any of save/validate/exit right now

Note that this is not supported for Java, as Java class doesn't have required flag. And also the required property on the JSON schema should be a string array, which contains a list of required field names, but not boolean flag in each properties as per JSON schema definition. Syndesis Salesforce connector will need a tweak.

[igarashitm]

kinda related to #116 , should we have save/validate button