[qt] Improve validation

[MartinDelille]

Description

Validation is missing in Qt client.

Swagger-codegen version

HEAD

Swagger declaration file content or url

https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/test/resources/2_0/petstore.yaml

Command line used for generation

$ swagger-codegen generate -i ../api.yml -l qt5cpp

Steps to reproduce

• Generate client
• Instance Pet object without setting the name
• See neither compilation nor execution error

Related issues/PRs

I discovered this issue after raising #7802

Suggest a fix/enhancement

A first step would be to add required parameter to the model constructor.

A second step would be to avoid property setter with pointer and add unsetter for optionnal field.

[MartinDelille]

Ping @ravinikam @stkrwork @fvarose

[wing328]

cc @etherealjoy as well