NavigationViewItem input handling fixes

[RBrid]

These changes address a series of bugs in the NavigationViewItem implementation:
#1 NavigationViewItem does not update its visual state when its IsSelected property changed.

• In WinUI3, often when you tap a NavigationViewItem, it stays dark gray.
• When you select a NavigationViewItem programmatically, it does not change its visual state.
  Fix: Listening to IsSelected change notifications to refresh visual state.

#2 PonterMoved does not update NavigationViewItem visual state in WinUI3.

• Move mouse cursor over NavigationViewItem A
• Tap NavigationViewItem B
• Move mouse cursor a bit --> NavigationViewItem A does not update its visual state.
  Fix: Listening to PointerMoved to update visual state when pointer is not thought to be over.

#3 Mouse pointer is not captured in PointerPressed.

• Mouse button down on NavigationViewItem.
• Move mouse outside of NavigationViewItem.
• Release mouse button --> NavigationViewItem does not update its NavigationViewItem visual state. It stays dark gray until you interact with it again.
  Fix: Pointer is captured in PointerPressed and released in PointerReleased, or when pointer is lost.

#4 IsEnabled change does not update NavigationViewItem visual state.
Fix: Listening to NavigationViewItem.IsEnabledChanged to clear the state flags and refresh visual state.

#5 Beginning of OnApplyTemplate does not reset template parts, or clear event handlers.
Fix: All control parts and handler are cleared at beginning of OnApplyTemplate to avoid leftover incorrect fields.

#6 Pointer input handlers that reset internal flags are not invoked when args.Handled is true causing bugs.
Fix: Input handlers that reset flags are not skipped thanks to the handledEventsToo=True feature.

#7 Multi finger inputs are not handled properly. Removing second finger clears pressed state for instance.
Fix: Only one pointer Id is tracked for a given NavigationViewItem now. Any newcomer is ignored.

o Also did some dead code cleanup, and code cleanup in general.
o Did some ad-hoc testing, expanded controls test app and NavigationView test.

(These changes also address 26317023 once pushed into WinUI)

[RBrid]

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[ojhad]

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[RBrid]

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