[NUI] Add SelectionHandleImage struct to Text Components

[NUI] Add SelectionHandleImage struct to Text Components

Add a SelectionHandleImage struct to pass data
of DALi SelectionHandleImageLeft, SelectionHandleImageRight PropertyMap.
The SelectionHandleImage struct can be used as an argument to
SetSelectionHandleImage, GetSelectionHandleImage, SetSelectionHandlePressedImage, GetSelectionHandlePressedImage,
SetSelectionHandleMarkerImage and GetSelectionHandleMarkerImage methods.

// example
var selectionHandleImage = new Tizen.NUI.Text.SelectionHandleImage();
selectionHandleImage.LeftImageUrl = "handle_downleft.png";
selectionHandleImage.RightImageUrl = "handle_downright.png";

field.SetSelectionHandleImage(selectionHandleImage);

Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>