Allow different ways to handle zoom and position between images

(Taken from bug #337262 )

Over the years, there has been much discussion about whether the zoom and position should be kept the same between images (see, for example, bug 293103, which I submitted, or bugs 291759, 294915, 321122, 327889, 331412, 334530, 337037 --- there may be more!).  I have even submitted a patch (with bug 293103) which was applied in modified form---thanks!  However, it has been subsequently broken...

I have come to realise that there are THREE main ways people like to have the zoom and position settings applied to successive images:

1. Each image should be opened in Zoom to Fit mode, even if the previous image was zoomed in or out and was panned to a different position.  This is Aurélien Gâteau's preferred mode of operation: call it Autofit zoom mode.

2. The zoom and position settings should be shared across all images.  New images should be opened with the previous image's settings.  If you go back to a previous im...