ProgressRing.IsActive should be true by default

`ProgressRing.IsActive` is driven by the `Visibility`, so when I change `Visibility`, then it always is set `IsActive == Visibility.Visible`. It's ok, just by default when I make

``` xml
    <mahapps:ProgressRing />
```

Then is `Visibility == Visible` and `IsActive == False`. To be consistent with the VisibilityChanged callback, it should be `IsActive == True` by default.
