Let the keyboard navigation work correctly in the Navigatioin controls.

[walterlv]

What does this PR do

Before this PR, the up key and the down key do not work when the Navigation view gets focused. And after this, you can press up and down to navigate focus among the navigation items, just like what the Windows Settings and the Microsoft Store do.

• Up/Down: Focus changes among the navigation items from the first to the last. The change stops when the first or the last has arrived.
• Tab/Shift+Tab: Focus will enter the navigation view only once and the next Tab will change the focus to other controls. If you want to switch focus inside it, use the Up/Down key instead.
• Ctrl+Tab: Not defined. It will behave in a default way which is changing the focus to the next and next until outside of the navigation view. If the user of the WPFUI package wants to switch it like a TabControl, he/she has to set the KeyboardNavigation.ControlTabNavigation attached property to Once manually.

What changed in this PR

1. OnKeyDown is overridden in the NavigationItem class. we handle the up and down keys to navigate focus states. If the navigation happens, the Handled property is set to true to prevent further event handling. (Reason: If we do not set the Handled to true, the ScrollViewer will handle it so that the KeyboardNavigation will not do further navigation. That is why we need to do the navigation ourselves but not using the native behavior that the KeyboardNavigation provides.)
2. OnKeyDown is overridden in the Navigation class. In most cases, this method does nothing because it does not receive focus by default. But if someone sets focus on it, the key handling can move the focus to its navigation children.
3. We override some metadata of KeyboardNavigation to set the default values of the navigation behavior. We set the directional navigation to Contained and the tab navigation to Once to make sure that the focus behavior of our Navigation works just like what the Windows Settings and the Microsoft Store do. We make the control-tab navigation to keep its default value because this behavior is hard to determine in a class library.

More information on the KeyboardNavigationMode enum can be found here:

• https://docs.microsoft.com/en-us/dotnet/api/system.windows.input.keyboardnavigationmode