Provides API to pass native handle of Tizen view to user

[bbrto21]

TizenViewBase has three top-level components depending on view types. and only if the view is TizenViewElementary, we provide a handle to the user via the Embedder API(FlutterDesktopViewGetEvasObject).

TizenView	top-level component
TizenWindowElementary	Evas_Object(elm_win)
TizenWindowEcoreWl2	Ecore_Wl2_Window
TizenViewElementary	Evas_Object(elm_table)

I think FlutterDesktopViewGetEvasObject should be renamed to a more generic, and improved to work properly for each view types.

Note: This requirement can affects flutter-tizen.

This is just my idea. If you have a better solution, please feel free to suggest.