Pd 0.52 event loop

Pd 0.52 with an (optional) Cocoa event loop to run VST plugin GUIs without disturbing audio processing.

NOTE: The event loop breaks externals like Gem or ophelia (because they expect to run on the main thread). To use these externals, deactivate the event loop by unchecking "Enable event loop" in the "Startup" settings or by starting Pd with "-noeventloop".