[Title] Heading size option in custom components

## Feature Request

**Problem**

When I create custom component that uses heading size option I want to reuse Core Componetns approach. I can easily add form select widget to my dialog with datasource from *AllowedHeadingElementsDataSourceServlet* and default values, but I need to provide
custom JavaScript that is responsible for hiding datasource or default values in dialog.
Moreover my component's policy requires **allowedHeadingElements** property even if author shouldn't have possibility to change allowed elements. (It can be solved with a hidden form resource type for that property)

**Solution**
1. Core Components provides generic JS to hide datasource or default heading size in dialog.
2. AllowedHeadingElementsDataSourceServlet always returns default, non-empty allowed heading elements. List may be configured in OSGI.

**Environment**
Core Components version 2.5.0

Start with AllowedHeadingElementsDataSourceServlet and the Core Components heading-size dialog behavior. Review how custom JavaScript hides the datasource or default heading values and how the allowedHeadingElements policy property is handled. Done means generic dialog behavior is reusable, and the servlet returns a configurable, non-empty default list.

Written by the indexing model from the issue text.