Zoom display (rebased onto dev_5_0)



This is the same as gh-2291 but rebased onto dev_5_0.

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This PR should display correctly nominal magnification in big image viewer (x40) rather then scale (100%).
In order to test this PR import big image that has AppMag set (for example SVS) and check if you see ```Magnification: x10```. Then go to big image that has been imported before changes in https://github.com/openmicroscopy/bioformats/pull/1007 and check if you still see ```Scale: 25%```