Stop the Sample app needing to be rebuilt even when nothing has chang…

…ed (#3558)

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## Fixes #3332
<!-- Add the relevant issue number after the "#" mentioned above (for ex: Fixes #1234) which will automatically close the issue once the PR is merged. -->

<!-- Add a brief overview here of the feature/bug & fix. -->

Stop the sample app from always triggering a rebuild, even if no code has been changed.

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

<!-- - Bugfix -->
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
 - Other... Please describe: Change in compilation behavior of the Sample App


## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->

VS will always rebuild the sample app when launched for debugging.

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->

Now VS will only rebuild the code if something has changed.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [ ] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [ ] Sample in sample app has been added / updated (for bug fixes / features)
    - [ ] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)
- [ ] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [ ] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [x] Contains **NO** breaking changes

**All unchecked options above are not applicable**

<!-- If this PR contains a breaking change, please describe the impact and migration path for existing applications below. 
     Please note that breaking changes are likely to be rejected within minor release cycles or held until major versions. -->


## Other information

Microsoft.Toolkit.Uwp.SampleApp/Microsoft.Toolkit.Uwp.SampleApp.csproj

@@ -1471,12 +1471,10 @@
     <Page Include="Styles\Custom\PivotHeaderItemUnderlineStyle.xaml">
       <Generator>MSBuild:Compile</Generator>
       <SubType>Designer</SubType>
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Page>
     <Page Include="Styles\Generic.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Page>
     <Page Include="Styles\GithubIcon.xaml">
       <SubType>Designer</SubType>
@@ -1485,7 +1483,6 @@
     <Page Include="Styles\Themes.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
-      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Page>
   </ItemGroup>
   <ItemGroup>