Replace batch/shell script with MSBuild

Build/postbuild_emuhawk.bat

@@ -1,3 +0,0 @@
-@CHDIR "%~dp0\.."
-@ROBOCOPY /E Assets output
-@EXIT 0

Build/postbuild_emuhawk.sh

@@ -1,3 +0,0 @@
-#!/bin/sh
-cd "$(dirname "$0")/.."
-cp -f -r Assets/* output || printf ""

src/BizHawk.Client.EmuHawk/BizHawk.Client.EmuHawk.csproj

@@ -640,7 +640,9 @@
 		<Compile Update="UpdateChecker.cs" SubType="Code" />
 	</ItemGroup>
 	<Target Name="PostBuild" AfterTargets="PostBuildEvent">
-		<Exec Command='"$(ProjectDir)..\..\Build\postbuild_emuhawk.bat"' Condition=" '$(OS)' == 'Windows_NT' " />
-		<Exec Command='"$(ProjectDir)../../Build/postbuild_emuhawk.sh"' Condition=" '$(OS)' != 'Windows_NT' " />
+		<ItemGroup>
+			<AssetFiles Include="$(ProjectDir)../../Assets/**/*.*" />
+		</ItemGroup>
+		<Copy SourceFiles="@(AssetFiles)" DestinationFolder="$(OutputPath)%(RecursiveDir)" SkipUnchangedFiles="true" />
 	</Target>
 </Project>