Add option to show exposure time as a fraction

[oskirby]

There is a debate raging between the three ways to represent exposure time, each with their own camp of users: time (us) for the scientific crowd, shutter angle for the cinema crowd, and fractions for the photographic crowd. The main window currently just always displays time and shutter angle, but we have been asked to provide a fraction for the exposure time too. Maybe this could be an option somewhere?