QuickTime and PDFKit demos

I have cleaned up the Cocoa bridge and started putting together a library of utility words in library/cocoa/. The WebKit demo is much shorter now because the bulk of its code has been moved to the library. I have put together two new demos, a QuickTime player and PDF viewer.

Both demos also use the new Cocoa event loop code, which integrates Cocoa event handling with Factor's multitasking. You can continue entering expressions in the listener and such while the Cocoa demos are running. However, you cannot run the SDL-based UI after one of these Cocoa demos. When the UI gets a Cocoa backend this will no longer be an issue.