Allow to define CSS classes for any element

[dontub]

This makes it possible for any control and layout to specify CSS classes in the UI
schema that will be added to the generated HTML code. The classes can be set at
the property cssClasses in the options property, e.g.:

{
  "type": "VerticalLayout",
  "elements": [
    {
      "type": "Control",
      "scope": "#/properties/name",
      "options": {
        "cssClasses": ["my-name-control-class"]
      }
    }
  ],
  "options": {
    "cssClasses": ["my-layout-class"]
  }
}

systopia-reference: 29920