Last improvements of the Page Builder API

[FabienLelaquais]

Description

A few things

• The renderers module is now visible, and exposes the classes BlockElementApi, ClassApi, ControlElementApi, DefaultBlockElement, and ElementApi.
  
  All these must be hidden. The one thing we want to document is the taipy.gui.Builder module, where the entirety of the Reference documentation will be stored.
• When invoking an element constructor (taipy.gui.Builder.element_name()), it would be very valuable to make the first parameter the default property of the element. For example, a text element could be instantiated with tgb.text("The text").
• We might want to add an html class to include raw HTML content:
  
  • tgb.html("h1", text="My title") would produce <h1>My title</h1>.

  •  with tgb.html("div"):
         tgb.text("The text")
    

    would produce <div><span ...>The text</span></div>
    This API would have an attributes parameter holding a dict that is used to set the element's HTML attributes.

Acceptance Criteria

• Ensure new code is unit tested, and check code coverage is at least 90%