Add NavigationViewItemSeparator lightweight styling theme resources

[Felix-Dev]

Description

This PR adds lightweight styling theme resources for the NavigationViewItemSeparator to enable easy customization of its default appearance without having to retemplate it. The following theme resources have been added (each using the current default values):

Foreground:

• TopNavigationViewItemSeparatorForeground
• NavigationViewItemSeparatorForeground

Margin:

• TopNavigationViewItemSeparatorMargin
• NavigationViewItemSeparatorMargin
• NavigationViewCompactItemSeparatorMargin

Thickness:

• TopNavigationViewItemSeparatorWidth
• NavigationViewItemSeparatorHeight

Motivation and Context

Closes #2139.

How Has This Been Tested?

Tested visually.

Screenshots (if appropriate):

Top mode

First separator from the left uses default styling, second separator uses the following theme resources:

<SolidColorBrush x:Key="TopNavigationViewItemSeparatorForeground" Color="Orange" />
<x:Double x:Key="TopNavigationViewItemSeparatorWidth">2</x:Double>
<Thickness x:Key="TopNavigationViewItemSeparatorMargin">2,0</Thickness>

Margin:

Left mode (expanded + compact)

First separator from the top uses default styling, second separator uses the following resources:

<SolidColorBrush x:Key="NavigationViewItemSeparatorForeground" Color="Red" />
<x:Double x:Key="NavigationViewItemSeparatorHeight">2</x:Double>
<Thickness x:Key="NavigationViewItemSeparatorMargin">32,2</Thickness>
<Thickness x:Key="NavigationViewCompactItemSeparatorMargin">8,2</Thickness>

Margin (same for both left expanded and left compact mode)

[ranjeshj]

@Felix-Dev can you please merge with master which includes some stability fixes for the PR pipeline ?

[Felix-Dev]

@ranjeshj Done.

[ranjeshj]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

🕐

[StephenLPeters]