Show zoom percentage in menu button

The zoom percentage is now shown next to the other zoom buttons. Also,
increase the zoom multiplier step to 1.1.
Disable zoom buttons/keys when no image is shown
Use average of w/h scale factors instead of max

This fixes a bug where scrollbars were appearing in "best fit" zoom mode
(because one of the image dimensions was 1 pixel too big). Also, use
nicer syntax for what is a necessary int-to-double cast.