Allow zooming on center image

[StableLlama]

Allow zooming in in the center widget image with control + mouse wheel. When zoomed in the image can be dragged around with the left mouse button. The middle mouse button toggles between zoom-to-fit and orignial size of the image (100% view).

[jhc13]

Hi, I am currently very busy, so I will review this later when I have more time.

[jhc13]

I think it would be better to have the buttons inside the image viewer, instead of in the main window toolbar, to keep things self-contained.

[StableLlama]

Can you make a mockup / screenshot what location you are preferring for these buttons?

Please note: as I wrote before, my final target is to add cropping and masking support. Then there will be more buttons that need a place. So using the toolbar for that is IMHO a good choice as the user can drag it to the place that suits best. Or even deactivate the toolbar to create more screen space when the shortcuts are used instead.

Also just for information: due to performance reasons the cropping/masking code required a change to use QGraphicsScene in the ImageViewer, as you can see live at https://github.com/StableLlama/taggui/tree/crop_editor

[StableLlama]

This PR is the basis of #353 and is made obsolete by it. You might pull this first and then #353 - or pull #353 directly.