RAR: add control over whether to copy .xml and/or .pdb to output

For `` items we should have more fine-grained control over whether to copy the related .pdb and .xml to output or not.

The .xml files actually don't need to be in the output directory because they are only useful for the IDE to show intellisense, and the IDE takes them directly from where the reference is first resolved. We only need to copy the XML file to output that comes from Csc.

Just turning off copying would be a breaking change (since builds may have validation that checks that the .xml files are in the output).

Proposal:

1. consider introducing an MSBuild property that controls whether to copy .xml files to output or not. Default it to true.
2. same for .pdbs I think (also default it to true).
3. Pass the properties to RAR so RAR can only include related files if that property is enabled: http://source.dot.net/#Microsoft.Build.Tasks.Core/AssemblyDependency/ReferenceTable.cs,2737
4. Consider adding item metadata "RelatedFilesToCopyToOutput" that can by default be RelatedFileExtensions: http://source.dot.net/#Microsoft.Build.Tasks.Core/AssemblyDependency/ResolveAssemblyReference.cs,23b76c72b911f0c9 This should give more fine-grained control per reference item (to override the properties set in 1 and 2)