Correct Default Application Selector styles for high contrast and to …

…change with OS theme dark/light toggle (#10185)

Correct Default Application Selector styles for high contrast and to change with OS theme dark/light toggle

## References
- https://docs.microsoft.com/windows/uwp/design/controls-and-patterns/xaml-theme-resources

## PR Checklist
* [x] Closes #10181
* [x] I work here
* [x] Manual tests passed

## Detailed Description of the Pull Request / Additional comments
1. If I'm going to override colors, I need to define styles in a resource dictionary with Light, Dark, and HighContrast variants so it can be appropriate for each of those. 
2. For HighContrast, I need to not mess with text colors and let them follow the default settings. 
3. For using System Brushes, I need to use a `ThemeResource` binding not a `StaticResource` binding. The former lets it change when you flip the OS toggle Light/Dark. The latter is stuck to whatever it was when the page loaded.

## Validation Steps Performed
- Loaded in light mode. Flipped to dark. Watched it change live. Checked both unselected and rollover/selected to ensure it was fine.
- Loaded in dark mode. Flipped to light. Watched it change live. Checked both unselected and rollover/selected to ensure it was fine.
- Flipped to HC. Watched it change live. Confirmed that unselected is black/white contrast and the roll over has the cyan/black. (No longer uses special second-line brush for HC, matches the controls I modeled this one on from OS Settings).

src/cascadia/TerminalSettingsEditor/CommonResources.xaml

@@ -11,6 +11,26 @@
         <ResourceDictionary Source="SettingContainerStyle.xaml" />
     </ResourceDictionary.MergedDictionaries>
 
+    <ResourceDictionary.ThemeDictionaries>
+        <ResourceDictionary x:Key="Light">
+            <Style x:Key="SecondaryTextBlockStyle"
+                   TargetType="TextBlock">
+                <Setter Property="Foreground" Value="{ThemeResource SystemBaseMediumColor}" />
+            </Style>
+        </ResourceDictionary>
+        <ResourceDictionary x:Key="Dark">
+            <Style x:Key="SecondaryTextBlockStyle"
+                   TargetType="TextBlock">
+                <Setter Property="Foreground" Value="{ThemeResource SystemBaseMediumColor}" />
+            </Style>
+        </ResourceDictionary>
+        <ResourceDictionary x:Key="HighContrast">
+            <Style x:Key="SecondaryTextBlockStyle"
+                   TargetType="TextBlock" />
+            <!--  Do not mess with the foreground color for High Contrast. Let it ride as is.  -->
+        </ResourceDictionary>
+    </ResourceDictionary.ThemeDictionaries>
+
     <x:Double x:Key="StandardIconSize">14.0</x:Double>
     <Thickness x:Key="StandardIndentMargin">13,0,0,0</Thickness>
     <Thickness x:Key="StandardControlMargin">0,24,0,0</Thickness>

src/cascadia/TerminalSettingsEditor/Launch.xaml

@@ -114,12 +114,12 @@
 
                                     <TextBlock Grid.Row="1"
                                                Grid.Column="1"
-                                               Foreground="{StaticResource SystemBaseMediumColor}"
+                                               Style="{ThemeResource SecondaryTextBlockStyle}"
                                                Text="{x:Bind Author}" />
 
                                     <TextBlock Grid.Row="1"
                                                Grid.Column="2"
-                                               Foreground="{StaticResource SystemBaseMediumColor}"
+                                               Style="{ThemeResource SecondaryTextBlockStyle}"
                                                Text="{x:Bind Version}" />
 
                                 </Grid>