#9 Fix UI rendering path

[Henning Thoele]

Currently there are issues when the overriden implementation of UIBaseComponent.Render does not call base.Render() correctly.

Acceptance criteria:

• Rewrite rendering to use two separate functions for walking the hierarchy and rendering the actual content.
• The actual rendering method shall be called "Draw", whereas the hierarchy walking method shall be called "InternalRender"

[Henning Thoele]

Integrated on develop: d52ef62