Sample App Heads need to be able to load libraries based on Platform

[michael-hawker]

Our sample app itself needs to be able to load different libraries across platforms in order to improve it. 🙂

Hmm... I'm having trouble even just trying to add a split reference to Microsoft.Toolkit.Uwp/CommunityToolkit.WinUI.UI.Controls.Markdown package in Labs.Heads.props, figured I could pull in the Labs.Uno.props file helpers as well and do something like:

  <!-- Project Head/Sample App dependencies -->
  <ItemGroup Condition="'$(IsUwp)' == 'True' AND '$(IsUno)' == 'False'">
    <PackageReference Include="Microsoft.Toolkit.Uwp.UI.Controls.Markdown" Version="7.1.2"/>
  </ItemGroup>
  <ItemGroup Condition="'$(IsWinAppSdk)' == 'True' AND '$(IsUno)' == 'False'">
    <PackageReference Include="CommunityToolkit.WinUI.UI.Controls.Markdown" Version="7.1.2"/>
  </ItemGroup>

(See branch llama/markdown)

But it's not restoring properly... hmm. May need some help here @Arlodotexe. In the meantime, I'm going to try and clean-up the MSB4011 warnings with an idea I had for #99

Originally posted by @michael-hawker in #151 (comment)

Needed for #151 (Toolkit Markdown Control) and #170 (Behaviors).
Related to #165 (but that's lower-priority atm).

We just need in our heads prop or some other location to be able to specify libraries that the Sample App shared project itself relies on for its chrome/display. These should be fairly minimal and get easier when we get to 8.0 for the main Toolkit (and not worried about consuming something from Labs at the moment, we probably want to avoid that).