gui viewport: make magnification easier to understand

Suggestion from Lennard.

The magnification is always the same (magical) number, indepedent of the number of
images displayed. If only one MPP, then also indicate the digital zoom.

That is more involved than it sounds because the text got bigger, so we need some
cleverness to automatically resize the textctrl.
gui viewport: hack to always have the merge slider icons in the correct order

On the SECOM (and Delphi), the EM image is set as last (=right) by the canvas,
so the merge slider icon should reflects that.