NavigationView: Add theme resources to enable easy flyout padding customization

[Felix-Dev]

Description

This PR adds two theme resources to the NavigationView to provide easy padding customization for

• the overflow menu in Top PaneDisplayMode and
• the NavigationViewItem children flyout in the hierarchical NavigationView

Motivation and Context

Closes #2718.

How Has This Been Tested?

Tested visually.

Screenshots:

The following XAML

<Application.Resources>
    <Thickness x:Key="TopNavigationViewOverflowMenuPadding">0,24</Thickness>
    <Thickness x:Key="NavigationViewItemChildrenMenuFlyoutPadding">0</Thickness>
</Application.Resources>

gives the following resulting look:

Overflow Menu in Top PaneDisplayMode

NavigationViewItem children flyout

For reference, here is how the flyout padding looks by default (children flyout shown here, same padding used in the overflow menu):

[StephenLPeters]

/azp run

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[StephenLPeters]