OSX: attempt to load UI (menubar) from a nib file

If the app is in a bundle, attempt to load the Main nib file specified in
the bundle's Info.plist. If the file can't be loaded, the stub menubar which
only shows the app name is used.
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 works more like on X11 now

Moved calling of the expose callback to NSView drawRect to match X11.

Change events test to do MakeCurrent in expose event callback, not
before.