[NUI] Add SetOutline, GetOutline to Text Components

[NUI] Add SetOutline, GetOutline to Text Components

Add a Outline struct to pass data of DALi Outline PropertyMap.
The Outline struct can be used as an argument to
SetOutline and GetOutline methods.

// example
var outline = new Tizen.NUI.Text.Outline();
outline.Width = 2.0f;
outline.Color = new Color("#45B39D");
label.SetOutline(outline);

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