Added the base background icon feature for dialogs

Added a dp called BackgroundVisual that's responsible for displaying an
overlay visual behind the contents of a dialog. Useful for showing a
warning icon for example.
Closes #1474 

Usage:
```
Canvas visual = (Canvas)App.Current.Resources["appbar_warning"];
await this.ShowMessageAsync("Error", "Connection error: unable to connect", backgroundViusal: new VisualBrush(visual));
```

If you're creating the dialog yourself:
```
SimpleDialog sd = new SimpleDialog();
sd.Title = "Error";
[...]
Canvas visual = (Canvas)App.Current.Resources["appbar_warning"];
sd.BackgroundVisual = new VisualBrush(visual);
[...]
```
![capture1](https://cloud.githubusercontent.com/assets/4404199/3944212/e020b9c0-25f1-11e4-9bdb-c1f6d4f4638f.PNG)