Alowing syncing appbar in hard coded panorama.

This pull request allowing syncing appbar, when panorama is hard coded in xaml (Because of bug, when selection is not updated when is filled from itemsource)

Example:
```
        <controls:Panorama
            Title="pan tests"
            x:Name="ViewModels"
            cal:Message.Attach="[Event SelectionChanged] = [Action SelectionChanged($this.SelectedIndex)];[Event Loaded] = [Action SelectionChanged($this.SelectedIndex)]">
            <controls:PanoramaItem
                DataContext="{Binding Items[0]}"
                cal:Bind.Model="{Binding}">
                <local:Item1View
                    cal:Bind.Model="{Binding}" />
            </controls:PanoramaItem>

            <controls:PanoramaItem
                DataContext="{Binding Items[1]}">
                <local:Item3View
                    cal:Bind.Model="{Binding}"  />
            </controls:PanoramaItem>

            <controls:PanoramaItem
                DataContext="{Binding Items[2]}">
                <local:Item2View
                    cal:Bind.Model="{Binding}"  />
            </controls:PanoramaItem>

            <controls:PanoramaItem
                DataContext="{Binding Items[3]}">
                <local:Item2View
                    cal:Bind.Model="{Binding}"  />
            </controls:PanoramaItem>

            <controls:PanoramaItem
                DataContext="{Binding Items[4]}">
                <local:Item4View
                    cal:Bind.Model="{Binding}"  />
            </controls:PanoramaItem>

        </controls:Panorama>
```
In the PanoramaOnSelectionChanged event _panoramaSelectedItem is PanoramaItem, instead of ViewModel at current index.

in SelectionChanged/Loaded I activate item at current index.