SL-33966: Viewer installed files are not described in a centralized source

This introduces the viewer_manifest.py and associated libraries.  Check out https://osiris.lindenlab.com/mediawiki/index.php/Installer_Manifest for some more information.

The gist of it is that all files to be included in the viewer installations are described in viewer_manifest.py.  viewer_manifest.py also acts as a packager, if you give it the right options.  All of the old methods of packaging (Makefile_Mac, scons, and the various .bat files) still work, but they are mostly just shells for calls to viewer_manifest.py.

You're now required to have python 2.3 or later on your machine to be able to package up an installer, but thankfully most people already have this.