Fix regression introduced in PR #36

[mast-eu]

In PR #36 I've overseen that 2 different targets in GitExtensions.PluginManager.csproj were copying the file PackageManager.UI.exe to different locations:

1. $(TargetDir)\PackageManager\PackageManager.UI.exe for building the NuGet package
2. $(GitExtensionsPluginsPath)\$(ProjectName)\PackageManager\PackageManager.UI.exe for local testing and debugging.

As a result, the second copy was erroneously removed.

The original targets (previous to #36) were:

gitextensions.pluginmanager/src/GitExtensions.PluginManager/GitExtensions.PluginManager.csproj

Lines 59 to 65 in 4d9433e

	<Target Name="CopyPackageManager" AfterTargets="ResolveReferences">
	<Copy SourceFiles="$(PackageManagerSourcePath)" DestinationFolder="$(TargetDir)PackageManager" />
	</Target>
	<Target Name="PostBuild" AfterTargets="PostBuildEvent">
	<Copy SourceFiles="$(TargetDir)$(PackageManagerTargetPath)" DestinationFolder="$(GitExtensionsDebugPluginsPath)PackageManager" />
	<Copy SourceFiles="$(TargetPath)" DestinationFolder="$(GitExtensionsDebugPluginsPath)" />
	</Target>

This PR now restores the second copy, in order to use the correct PackageManager.UI.exe locally for debugging and testing.