Fix issue with treeview icon on high contrast (#1308)

<!--- Provide a general summary of your changes in the Title above -->

## Description
<!--- Describe your changes in detail -->
Fixes issues with one icon not being visible on the TreeView page on
high contrast.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here. -->
Fixes #648

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>

WinUIGallery/ContentIncludes.props

@@ -13,7 +13,6 @@
     <Content Include="Assets\ControlIcons\DefaultIcon.png" />
     <Content Include="Assets\ControlIcons\DropdownButtonIcon.png" />
     <Content Include="Assets\ControlIcons\EasingFunctionsIcon.png" />
-    <Content Include="Assets\file.png" />
     <Content Include="Assets\folder.png" />
     <Content Include="Assets\ControlIcons\MenuBarIcon.png" />
     <Content Include="Assets\ControlIcons\NavigationViewIcon.png" />

WinUIGallery/ControlPages/TreeViewPage.xaml

@@ -1,4 +1,4 @@
-<Page x:Class="AppUIBasics.ControlPages.TreeViewPage"
+<Page x:Class="AppUIBasics.ControlPages.TreeViewPage"
       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       xmlns:local="using:AppUIBasics"
@@ -24,9 +24,8 @@
         <DataTemplate x:Key="FileTemplate"
                       x:DataType="controlpages:ExplorerItem">
             <TreeViewItem AutomationProperties.Name="{x:Bind Name}">
-
                 <StackPanel Orientation="Horizontal">
-                    <Image Width="20" Source="../Assets/file.png"/>
+                    <FontIcon Glyph="&#xE8A5;" />
                     <TextBlock Margin="0,0,10,0"/>
                     <TextBlock Text="{x:Bind Name}"/>
                 </StackPanel>

WinUIGallery/ControlPagesSampleCode/TreeView/TreeViewTemplateSelectorSample_xaml.txt

@@ -13,7 +13,7 @@
     <DataTemplate x:Key="FileTemplate" x:DataType="controlpages:ExplorerItem">
         <TreeViewItem AutomationProperties.Name="{x:Bind Name}">
             <StackPanel Orientation="Horizontal">
-                <Image Width="20" Source="../Assets/file.png"/>
+                <FontIcon Glyph="&#xE8A5;" />
                 <TextBlock Margin="0,0,10,0"/>
                 <TextBlock Text="{x:Bind Name}"/>
             </StackPanel>