[NUI] Add SetUnderline, GetUnderline to Text Components

[NUI] Add SetUnderline, GetUnderline to Text Components

Add a Underline struct to pass data of DALi Underline PropertyMap.
The Underline struct can be used as an argument to
SetUnderline and GetUnderline methods.

// example
var underline = new Tizen.NUI.Text.Underline();
underline.Enable = true;
underline.Color = new Color("#3498DB");
underline.Height = 2.0f;
label.SetUnderline(underline);

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