Custom specialized question types do not inherit properties from the base question type

[RomanTsukanov]

When it comes to custom question types, we provide two options:

1. Create a specialized question type.

• Properties listed in questionJSON are hardcoded and cannot be edited.
• The custom question type is respresented in the UI (type selector, etc.) as a genuine question type.
• Properties of the base class are not inherited.

2. Add a custom toolbox item.

• Properties listed in questionJSON can be edited in Survey Creator.
• The custom question type looks like the question type it's based upon.
• Possesses all the properties of the base class.

However, there's middle-ground option, where questionJSON properties are hardcoded but properties of the base class are inherited. I think we should extend option 1 and let the user decide whether to inherit base class properties.

Survey.ComponentCollection.Instance.add({
  name: "country", 
  title: "Country", 
  defaultQuestionTitle: "Country",
  questionJSON: {
    "type": "dropdown",
    "placeholder": "Select a country...",
    "choicesByUrl": {
      "url": "https://surveyjs.io/api/CountriesExample",
    }
  },
  inheritBaseProps: true // inherits properties of the Dropdown question
});