Bugfixes in windows phone user control and samples

In the sample project when you try to enter for second time in the custom layout there was an exception, because the custom layer is never detached from the previous ScanPage.xaml

this was the exception
```
'gridCustomOverlay.Children.Add(CustomOverlay)' threw an exception of type 'System.InvalidOperationException' base: {System.InvalidOperationException: Element is already the child of another element.
at MS.Internal.XcpImports.CheckHResult(UInt32 hr)
at MS.Internal.XcpImports.Collection_AddValue[T](PresentationFrameworkCollection`1 collection, CValue value)
at MS.Internal.XcpImports.Collection_AddDependencyObject[T](PresentationFrameworkCollection`1 collection, DependencyObject value)
at System.Windows.PresentationFrameworkCollection`1.AddDependencyObject(DependencyObject
value)
at System.Windows.Controls.UIElementCollection.AddInternal(UIElement
value)
at System.Windows.PresentationFrameworkCollection`1.Add(T value)}
```
In the sample.

The action to the buttons (cancel and torch) of the custom layer were only added once, because the buttons where already wired with an action.

All the events in ScanPage are set to null when it's navigated from the
page, because the Events are static and we were adding action everytime
we navigated to the page