Add Labels to some Property groups

Some property groups have conditions that also self-explain their purpose.
So, Add labels to bare property groups only to differentiate among themselves.
Then, when contributors add any additional properties, they'll know where to put them.

Author: Nirmal4G

eng/Toolkit.Common.props

@@ -1,12 +1,12 @@
 <Project>
 
-  <PropertyGroup>
+  <PropertyGroup Label="Build">
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
     <ContinuousIntegrationBuild>$(TF_BUILD)</ContinuousIntegrationBuild>
     <SourceLinkEnabled>$(ContinuousIntegrationBuild)</SourceLinkEnabled>
   </PropertyGroup>
 
-  <PropertyGroup>
+  <PropertyGroup Label="Compile">
     <Nullable>Enable</Nullable>
     <LangVersion>11.0</LangVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
@@ -21,7 +21,7 @@
     <NoWarn>$(NoWarn);CS8500</NoWarn>
   </PropertyGroup>
 
-  <PropertyGroup>
+  <PropertyGroup Label="Assembly">
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)toolkit.snk</AssemblyOriginatorKeyFile>
     <AssemblySignPublicKeyFile>$(MSBuildThisFileDirectory)toolkit.spk</AssemblySignPublicKeyFile>

src/Directory.Build.props

@@ -3,7 +3,7 @@
   <Import Project="..\$(MSBuildThisFile)" />
 
   <!-- Common properties for all projects under src folder -->
-  <PropertyGroup>
+  <PropertyGroup Label="Build">
     <IsPackable>true</IsPackable>
     <IsPublishable>true</IsPublishable>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>

tests/Directory.Build.props

@@ -7,13 +7,13 @@
       This also includes non-test projects as well. But if it's required then
       override the following properties only within those projects.
   -->
-  <PropertyGroup>
+  <PropertyGroup Label="Build">
     <IsPackable>false</IsPackable>
     <IsPublishable>false</IsPublishable>
     <NoWarn>$(NoWarn);CS8002;SA0001</NoWarn>
   </PropertyGroup>
 
-  <PropertyGroup>
+  <PropertyGroup Label="Project">
     <IsTestProject>true</IsTestProject>
     <!-- The MVVM Toolkit unit tests use `Mvvm.ExternalAssembly` project as a `ProjectReference` for testing -->
     <IsTestProject Condition="$(MSBuildProjectName.Contains('Mvvm.ExternalAssembly'))">false</IsTestProject>