Support for Map model property type

[ghost]

As a Java developer, I want to set a Map<Number|Boolean|String, Number|Boolean|String|Bean> property type so that I can easily have an object property on the client-side with arbitrary key/value pairs. For example:

{
  "someKey": someValue,
  "someOtherKey": someOtherValue
  ...
}

The current workaround for this limitation is to manually copy the Map into a JsonObject and set this via getElement().setPropertyJson(name, jsonObject)

Conversation: https://vaadin.slack.com/archives/C6RAXJATF/p1504876959000295

[pleku]

Same issue for template model #738

[mshabarov]

Implemented in #7308.