WindowsPlugin: support multiple .wxs inputs

[nigredo-tori]

This would be useful when working with custom WiX scripts - for example, when harvesting a directory using heat. Since heat generates a separate .wxs file, the only option to use it with the current packageBin implementation would be transforming it to an include file, which is incredibly ugly, and would require working around the copy logic.

Both candle and light support using multiple input files, so implementing this feature can be as simple as replacing wixFile: TaskKey[File] with wixFiles: TaskKey[Seq[File]], and updating packageBin implementation accordingly. If there's a need to preserve compatibility with the code that uses wixFile, we can keep wixFile and just do this:

wixFiles := List(wixFile.value)

[muuki88]

Thanks for this feature request. Would you like to make a pull request for this?