Add new 'DesignTools' projects (#3318)

## Fixes #3252 

Created the `DesignTools` projects and refactored the code to work with the new XAML Designer.
Both `Design` and `DesignTools` projects live SxS (_Side by Side_) now, so that, we can ship for both the designers for the foreseeable future.

_More info in each of the commits' description._

## PR Type
What kind of change does this PR introduce?

- Bugfix
- Feature
- Refactoring (no functional changes)
- Build

## What is the current behavior?
Design tools don't work well with our controls (after VS v16.4 upgrade) because of the new XAML designer.


## What is the new behavior?
Now the design tools should work as before as the new `DesignTools` projects have been introduced to work with the new XAML designer.


## PR Checklist

Please check if your PR fulfills the following requirements:

- [x] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](../contributing.md#docs). Link: **NIL**
- [x] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [x] Contains **NO** breaking changes

## Other information
More info will be added as the PR progresses

Author: msftbot[bot]

Directory.Build.props

@@ -14,8 +14,9 @@
     <IsTestProject>$(MSBuildProjectName.Contains('Test'))</IsTestProject>
     <IsUwpProject Condition="'$(IsDesignProject)' != 'true'">$(MSBuildProjectName.Contains('Uwp'))</IsUwpProject>
     <IsSampleProject>$(MSBuildProjectName.Contains('Sample'))</IsSampleProject>
-    <DefaultTargetPlatformVersion>19041</DefaultTargetPlatformVersion>
-    <DefaultTargetPlatformMinVersion>17763</DefaultTargetPlatformMinVersion>
+    <TargetPlatformBaseVersion>10.0</TargetPlatformBaseVersion>
+    <TargetPlatformRevision>19041</TargetPlatformRevision>
+    <TargetPlatformMinRevision>17763</TargetPlatformMinRevision>
     <PackageOutputPath>$(MSBuildThisFileDirectory)bin\nupkg</PackageOutputPath>
   </PropertyGroup>
 
@@ -88,7 +89,6 @@
   </Choose>
 
   <PropertyGroup Condition="'$(IsUwpProject)' == 'true'">
-
     <!-- 8002 is a strong named -> non-strong-named reference -->
     <!-- This is valid for platforms other than .NET Framework (and is needed for the UWP targets -->
     <NoWarn>$(NoWarn);8002</NoWarn>
@@ -103,4 +103,7 @@
       <Link>stylecop.json</Link>
     </AdditionalFiles>
   </ItemGroup>
+
+  <Import Project="$(MSBuildThisFileDirectory)build\Windows.Toolkit.VisualStudio.Design.props" Condition="'$(IsDesignProject)' == 'true'"/>
+
 </Project>

Directory.Build.targets

@@ -1,33 +1,20 @@
 <Project>
-  <Choose>
-    <When Condition="'$(TargetFramework)' == 'uap10.0' or '$(TargetFramework)' == 'uap10.0.17763' or '$(TargetFramework)' == 'native' or '$(TargetFramework)' == 'net461'">
-      <!-- UAP versions for uap10.0 where TPMV isn't implied -->
-      <PropertyGroup>
-        <TargetPlatformVersion>10.0.$(DefaultTargetPlatformVersion).0</TargetPlatformVersion>
-        <TargetPlatformMinVersion>10.0.$(DefaultTargetPlatformMinVersion).0</TargetPlatformMinVersion>
-      </PropertyGroup>
 
-      <PropertyGroup Condition="'$(DebugType)' == ''">
-        <DebugType>Portable</DebugType>
-      </PropertyGroup>
+  <PropertyGroup>
+    <UseUWP Condition="'$(TargetFramework)' == 'uap10.0' or '$(TargetFramework)' == 'uap10.0.17763' or '$(TargetFramework)' == 'native' or '$(TargetFramework)' == 'net461'">true</UseUWP>
+  </PropertyGroup>
 
-      <ItemGroup>
-        <SDKReference Condition="'$(UseWindowsDesktopSdk)' == 'true' " Include="WindowsDesktop, Version=$(TargetPlatformVersion)">
-          <Name>Windows Desktop Extensions for the UWP</Name>
-        </SDKReference>
-      </ItemGroup>
-    </When>
-  </Choose>
-  
   <Choose>
-    <When Condition="'$(TargetFramework.Contains(`uap10.0`))' == 'false' and '$(TargetFramework)' != 'native' and '$(IsSampleProject)' != 'true'">
+    <When Condition="!$(TargetFramework.Contains(`uap10.0`)) and '$(TargetFramework)' != 'native' and '$(IsSampleProject)' != 'true'">
       <PropertyGroup>
         <SignAssembly>true</SignAssembly>
         <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)toolkit.snk</AssemblyOriginatorKeyFile>
       </PropertyGroup>
     </When>
   </Choose>
 
+  <Import Project="$(MSBuildThisFileDirectory)build\Windows.Toolkit.Uwp.Build.targets" Condition="'$(UseUWP)' == 'true'"/>
+
   <Target Name="AddCommitHashToAssemblyAttributes" BeforeTargets="GetAssemblyAttributes">
     <ItemGroup>
       <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute" Condition=" '$(SourceRevisionId)' != '' ">
@@ -36,4 +23,5 @@
       </AssemblyAttribute>
     </ItemGroup>
   </Target>
+
 </Project>