Feedback from handled events

[bvssvni]

See PistonDevelopers/turbine#30

Need a way to detect whether events have been handled by the Ui.

[mitchmindtree]

I just added a reply to PistonDevelopers/turbine#30 and I've added some related issues:

• Get the widget currently under the mouse if there is one Provide a Ui method that returns whether or not the mouse is over a Widget. #611.
• Get whether or not user input is currently being captured Provide Ui methods that indicate whether or not user input is currently being captured. #612.
• Tell conrod when user input is captured or uncaptured by an external source Provide Ui methods that allow for user input to be captured externally. #613.