[NUI.Gadget] Add a new internal API (#4976) (#5064)

[NUI.Gadget] Add a new internal API (#4976) (#5064)

* [NUI.Gadget] Add new internal APIs

If the UI gadget developer sets the class name, the application developer
can adds a new NUIGadget instance using the NUIGadgetInfo object.

The following APIs are added:
 + NUIGadgetManager.Add(NUIGadgetInfo gadgetInfo); [method]
 + NUIGadgetManager.GetGadgetInfos(); [method]
 + NUIGadgetInfo.ClassName; [property]

* Change the visibility of the ClasaName property

The visibility of the NUIGadgetInfo.ClassName property is changed to internal.

* Modify description of GetGadgets()

* Remove Add() method

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Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
Co-authored-by: Jay Cho <chojoong@gmail.com>