[NUI] Add SetShadow, GetShadow to Text Components

[NUI] Add SetShadow, GetShadow to Text Components

Add a Shadow struct to pass data of DALi Shadow PropertyMap.
The Shadow struct can be used as an argument to
SetShadow and GetShadow methods.

// example
var shadow = new Tizen.NUI.Text.Shadow();
shadow.Offset = new Vector2(3, 3);
shadow.Color = new Color("#F1C40F");
shadow.BlurRadius = 4.0f;
label.SetShadow(shadow);

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