DUI: hide little circle.

# Preface
Little circle between query and actions should be presented only when both of them are presented.
# Solution
I used MultiBinding.
# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4488653/b7bb5e98-4a10-11e4-9bb7-8a9c4b7dac1f.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4488682/1f45c3c8-4a11-11e4-8f9e-7f5e26cacfdb.png)
# Reviewers
@Benglin,@vmoyseenko, please take a look.
Very nice trick, @aosyatnik! Since your pull request cannot be merged in right now, I'd request some minor changes :)

```cs
public class MultiBoolToVisibilityConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        bool anyTrue = false;
        foreach (var value in values)
            anyTrue = anyTrue || ((bool)value);

        return anyTrue ? Visibility.Visible : Visibility.Collapsed;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}
```