Adding 'Open File' and 'Open File Location' to PipelineTool tree context menu.

Adding 'Open File' and 'Open File Location' to PipelineTool tree context menu.
Added IProjectItem.OriginalPath.
- For PipelineProject this replaces FullPath
- For ContentItem this replaces SourceFile
- For FolderItem this is identical to Location
Added IController.GetFullPath(string) which returns the an absolute path given a project relative path (or returns unchanged path if already rooted).