Zooming Non-Intuitively Moves Image

[AldaronLau]

https://github.com/ArturKovacs/emulsion/issues/178

There should be a GUI toggle for turning on and off whether zooming uses the mouse position to move the image around, or fix the position while zooming. This preference should persist across program restarts.

Related, there should be a keyboard shortcut for zooming in and out (either Ctrl +, Ctrl -, or +, -)

[AldaronLau]

With further thought, I'm not sure the toggle is such a good idea. The addition of keyboard shortcuts that don't take into account mouse position are probably sufficient, while keeping scroll to zoom functionality consistent with how it is now. Although maybe still add ctrl+scroll as an alternative.