Enhancement: Add initial data support for checkbox

[saicheck2233]

Description: Add initial data support for checkbox so data can be initialized upon generation of the form. Below is the schema used:
Schema:

{
  "$schema": "http://json-schema.org/draft-03/schema#",
  "type": "object",
  "properties": {
    "animal": {
      "type": "boolean",
      "title": "Animals",
      "format": "checkbox",
      "enum": ["Cat", "Dog", "Mouse"],
      "description": "Checkbox with initial data"
    }
  }
}

Initial data:

{
  "animal": ["Dog", "Mouse"]
}

Pic before changes:


Initial data is defined but doesn’t reflects on the generated form.

Pic after changes:

Now the value defined in the initial data schema is ticked.