OSX: Fixed object management at window destruction

Keep around pointers to the window delegate and view and release separately
when the window is destroyed. The reason for that is that NSWindow close
does not destroy manually set delegates nor views (the objects might still
be needed elsewhere).
OSX: expose callback now resembles X11

Moved calling of the expose callback to NSView drawRect to match X11.
glwtGetTime implementations for win32, osx & posix
Updated events test to work with multiple windows.

Press return to create a new window (up to 16 windows supported)
Events now print timestamps.