v3 Add group visibility control

[muschottkey]

I would like to be able to control the visibility of the group. This has already been implemented in a pending PR in master https://github.com/vue-generators/vue-form-generator/pull/606

Added it to v3.

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

It introduces ability to show and hide entire groups.

• What is the current behavior? (You can also link to an open issue here) Groups can't be toggled.

• What is the new behavior (if this is a feature change)? Groups can be toggled in the same way fields are toggled. A visible attribute on the group schema is all that is needed.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) NO.

[coveralls]

Coverage decreased (-0.1%) to 73.795% when pulling a84221a2119b91b25c0cc43e460424ff7661da84 on muschottkey:v3-enable-group-visibility into 218bababcdb4dced2fb439d6f6644c23afbf2d16 on vue-generators:v3.