[C++] [Qt5] Add enum support for client and server

Created by: etherealjoy

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

• add support for enum for Qt5 client and server
• extend tests for Qt5 client

Spec used for testing client and server

---
# This is a sample Swagger spec, describing a simple API as a starting point.
swagger: "2.0"
info:
  description: Template Description
  version: 1.0.0
  title: Testspec


host: 0.0.0.0:8080
basePath: /api
schemes:
- http

paths:
  /v1.0/testintenum:
    get:
      description: ""
      operationId: testintenum
      responses:
        200:
          description: "Successful response"
          schema:
            $ref: '#/definitions/intEnum'
        400:
          description: "The request was not successfully executed."

  /v1.0/teststringenum:
    get:
      description: ""
      operationId: teststringenum
      responses:
        200:
          description: "Successful response"
          schema:
            $ref: '#/definitions/stringEnum'
        400:
          description: "The request was not successfully executed."

  /v1.0/testintenum1:
    get:
      description: ""
      operationId: testintenum1
      responses:
        200:
          description: "Successful response"
          schema:
            type: object
            additionalProperties:            
              $ref: '#/definitions/intEnum'
        400:
          description: "The request was not successfully executed."

  /v1.0/teststringenum1:
    get:
      description: ""
      operationId: teststringenum1
      responses:
        200:
          description: "Successful response"
          schema:
            type: array
            items:
              $ref: '#/definitions/stringEnum'
        400:
          description: "The request was not successfully executed."          

  /v1.0/test:
    get:
      description: ""
      operationId: test
      responses:
        200:
          description: "Successful response"
          schema:
            $ref: '#/definitions/enumeration'
        400:
          description: "The request was not successfully executed."

definitions:
  stringEnum:
    description: "string enum"
    enum:
      - abc
      - def
      - ghi
    type: string

  intEnum:
    description: "integer enum"
    enum:
      - 1
      - 2
      - 3
    type: integer

  enumeration:
    type: object
    description: "string enum"
    properties:
      enint:
        $ref: '#/definitions/intEnum'
      enstr:
        $ref: '#/definitions/stringEnum'
      enarr:
        type: array
        items:
          $ref: '#/definitions/stringEnum'
      enaobj:
        type: object
        additionalProperties:
          $ref: '#/definitions/intEnum'     

cc: @stkrwork @MartinDelille @fvarose @ravinikam