Image Viewer improvements

Kevin Funk requested to merge work/kfunk/imageviewer-improvements into master
showimagewidget: Use a QGraphicsView

Several advantages:
- Scaling can be handled by QGV itself
- We can scroll the image by mouse cursor
- Scaling is now done based on where the cursor is

testshowimage_gui: Allow to pass file name

... and whether the image is animated or not

showimagewidget: Add "Fit to View" button

Also reuse some more code where possible

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