WIP Pipeline GUI for Mac

This is an incomplete version of the Pipeline GUI for Mac OS X using the Xwt toolkit.  It can currently
- import XNA content projects
- load MGCB files
- save MGCB files
- add existing content files
- build content

What is missing
- a working treeview. It currently throws NullReferenceException.
- a working property grid. An initial implementation is present, but it is just a start.
- top level menus don't have any text, but they are there.
- dialogs for creating new content files.
- dialog for managing assembly references.
- many other features.

It may be a better option to rewrite using the native Cocoa UI.  Xwt has many limitations and may not be enough for us to complete the Pipeline GUI to a satisfactory level.