Do not handle events in work space when handled by the Ui

[bvssvni]

Currently the events are handled by both Ui and work space.

When clicking in the 3D scene, an entity is added at the ground. When clicking on the "reset" button, turbine tries to insert an entity. Needs feedback from Conrod of which events that were handled.

[mitchmindtree]

Hey @bvssvni just saw this :)

It sounds like it might be useful to add a method to the Ui that tells the user whether or not the mouse is currently over a widget, returning Some(widget::Index) if it is, or None if it is not.

It might also be useful to provide a method that indicates whether or not user input is currently being captured by conrod, as well as a way to tell conrod that user input has been captured/uncaptured by an external source.

I've added issues for these here, here and here. Let me know your thoughts! 😸