Add a Tooltip UI Element

Adds a tooltip UI Element which is used in the following way (from the editor (#61 commit later possibly)):

    Tooltip@ tooltip = Tooltip("Tooltip");
    tooltip.delay = 2000.f;

    Text@ text = Text();
    text.SetFont(font, 11);
    text.color = Color(0,255,0);
    text.text = tooltiptext;
    
    BorderImage@ background = BorderImage();
    background.size = text.size;
    background.color = Color(0,0,0);

    tooltip.AddChild(background);
    tooltip.AddChild(text);    
    
    element.AddChild(tooltip);

Which produces this:

![image](https://f.cloud.github.com/assets/2347963/1818523/d8209490-700a-11e3-92a3-2014b106ed84.png)

It work's off its parent element being hovered over and then displaying when the delay is reached. I decided not to create a heavyweight implementation forcing text and background as this way it provides more flexible usage for elements that already have some hover mechanism.

There might be a slight issue with this though due to BringToFront not working as expected (I think im missing something there) in that it is still behind some elements in my editor test.

![image](https://f.cloud.github.com/assets/2347963/1818524/fed64cce-700a-11e3-987c-6feb96a7582c.png)