Castle Game Engine: Easily load UI in TCastleControl using TCastleControl.DesignUrl

From Castle Game Engine: link to original post

I added TCastleControl.DesignUrl and TCastleControl.DesignedComponent to easier load designed UI in TCastleControl, and even see the designed UI in Lazarus form inspector.

The short documentation is added to TCastleControl chapter.

Our engine examples examples/lazarus/model_3d_with_2d_controls and examples/lazarus/two_controls use it already.