Huge amount of refactoring to use Cocoa here:

- Updated to .xib format.  To produce a new nib (which is required after changing the xib file), use "ibtool SecondLife.xib --compile SecondLife.nib" within the newview directory.
- xib file now defines the viewer's window.  VIews are still being pragmatically added to the main window.  This may change in the future.
- LLAppViewer's main loop has been slightly refactored to be executed on a timer for OS X.  This probably needs a bit more work.
- Event handling still needs more work to work within a timer based event loop.  It works fairly sporadically at the moment, as if events are being dropped between timer executions, at least for the mouse.
- Carbon has been purged from the viewer's startup, and from LLWindow entirely.  There's likely still a few odds and ends fragmented throughout the viewer and its dependencies.  Need to track these down.
- LLAppViewerMacOSX now uses NSApplicationMain, and also implements the NSApplicationDelegate protocol in LLAppDelegate.
- Fullscreen support has been implemented for OS X 10.7+
This is still a work in progress, however at this stage everything needed for a functional viewer is (mostly) complete.  From here, it's mostly just bug hunting and fixing.