Improve screenshot view zooming

a93b9cb4: Don't smooth at 200%

200% is large enough to use unsmoothed graphics

5e702ee5: 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.

8d80df9c: Zoom in/out in 0.25 steps if zoom < 1, 0.5 if < 2, 1 if >= 2

This makes the graphics look crisp more often, especially above 2x zoom.

490fb2d2: Increase spinbox step size to 25

zoom with 25% increments via the spinbox.

3ec8c2b4: Use 1 MouseArea to change the cursor instead of 2, fix cursor after drag

We didn't need 2 MouseAreas to do what we were trying to do and using containsPress instead of press makes the cursor shape go back to an open hand after a drag. Before, it would get stuck on the closed hand until you clicked on the mouse area again.