Magnifying glass for fitsviewer.

This MR adds a magnifying glass to the fitsviewer and fitsview windows in align, focus & guide.  Here's a screenshot: https://photos.app.goo.gl/zGFBMx8Hk9YgDxsj9

FWIW, the magnification is 8X whatever is currently displayed. Let me know if you think that amount is good, vs more or less magnification.

Putting it together was straightforward, but I wasn't 100% sure about the UI (e.g. how the user should enable & disable it). As a first shot, I used a control-key modifier when mousing over. That is, to use the magnifier:

• open an image in a fitsviewer,
• mouse over the image to a star, no need to press any mouse buttons,
• then press control to enable the magnifier (you may need to slightly move the mouse after you do that as the event is triggered on mouse motion).

Another possibility is enabling the magnifier by holding the mouse button down, but that action is currently used for dragging the scrollbar (magnify could replace that?). 

We could also add a floating button to enable/disable the magnifier for the view windows, and a similar pushbutton couple be placed atop the fitsviewer. Perhaps submit as is, and then following up with some UI changes to enable/disable magnification, if desired?

I tested on a mac and a raspberry pi 4, and both were sufficiently responsive using images from my 16-megapixel ASI1600. It's possible that slower machines or very large images may run slowly, but then again, the feature need not be used in those situations.