On Mac, move SLPlugin executable to the same directory as the viewer executable.

Change LLDir_Mac::getLLPluginLauncher() to look in the viewer's executable dir
instead of in the plugins dir. Change viewer_manifest.py's
DarwinManifest.construct() to put SLPlugin in the new location.
SLPlugin is being linked with our new libllcommon.dylib, which self-identifies
as being findable via @executable_path/../Resources/libllcommon.dylib. This
doesn't work from the Resources/llplugin subdir -- the above relative path
ends up looking in the nonexistent Resources/Resources subdirectory. Putting
SLPlugin in the Contents/MacOS directory with the viewer executable solves the
problem.