Public methods and public widgets for CustomNavigationPane

[Arenukvern]

Is your feature request related to a problem? Please describe.
First - thank you very much for the awesome library!

A problem:

NavigationPane has an option to provide customPane implementation.
Imagine you need to customize _TopNavigationPane by creating CustomTopNavigationPane.

Currently, if you try to implement it, you will have to copy the whole _MenuFlyoutPaneItem, which also depends from private PaneItem. _getPropertyFromTitle and NavigationPane._changeTo so you will have to copy it too.

Describe the solution you'd like
Make as public members the following widgets and methods:

PaneItem. _getPropertyFromTitle
_MenuFlyoutPaneItem
_NavigationViewScrollBehavior
NavigationPane._changeTo
_kCompactNavigationPanelWidth
_OpenNavigationPane
_kOpenNavigationPanelWidth
_CompactNavigationPane
_TopNavigationPane
add _overlayKey to NavigationPaneWidgetData

NavigationViewState.PaneScrollConfiguration -> separate widget
export package:fluent_ui/src/utils/popup.dart to library

Describe alternatives you've considered

Additional context

If that's okay, I would like to make these changes and add PR for it.
Will glad for your thoughts!

Please see a draft PR below:
#366 (comment)