Provide `Ui` methods that indicate whether or not user input is currently being captured.

The method would return `None` if the input is not captured, otherwise returning `Some(widget::Index)` with the index of the widget currently capturing the input. I.e.

``` rust
if ui.is_mouse_captured().is_none() {
    // Do stuff with mouse
}

if ui.is_keyboard_captured().is_none() {
    // Do stuff with keyboard
}
```
