Improve screenshot view zooming

This fixes multiple issues:

- Images smaller than the flickable would start at 100% zoom when annotating, but if you tried to zoom out to 100% after zooming in or even while already at 100%, the image would zoom to fit the flickable instead.
- The spinbox showed incorrect values with images smaller than the flickable.
- The zoom change from switching to annotation mode was not animated for images smaller than the flickable.
- When zooming in with the mouse wheel, it didn't alway zoom into the point where the mouse was located. It does that much more reliably now.
- Large images couldn't always perfectly zoom all the way out, leaving a useless scrollbar visible.

Removed zoomByFactor(). It was no longer needed.