Pinch-to-zoom

[angstsmurf]

Making the text bigger and smaller with the ⌘+ and ⌘- shortcuts works reasonably well, but it won't enlarge images, and it would be nice if we could use the built-in NSScrollView pinch-to-zoom magnification for this. It turns out to be non-trivial to get this to work with a NSTextView inside the scroll view, and when I tried to implement it I kept getting crashes.