[ENH] ImageViewer: Add a 'Preview' like window

[ales-erjavec]

Introduce a 'OSX Preview' like window to step through the images.

[codecov-io]

Current coverage is 88.19% (diff: 100%)

Merging #1402 into master will not change coverage

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[BlazZupan]

This is a great addition to Image Viewer. Has problems though if the images are very big (say 3000 x 3000). Then the preview window would cover the entire area of the screen. I can make preview window smaller, but as soon as I click on the main Image Viewer (or change the image by using arrow keys) the preview comes into focus in a full size, obscuring everything else.

This could be fixed with:

• preview window having the same size (does not scale automatically to the size of the image)
• preview can be made bigger or smaller, by the user, but stays the same with all the images
• preview window remembers its position and size

When preview opens, it shows at front (correct), but then I am not able to click on other windows, say on Image Viewer. It would be better if Preview is treated just as a regular window that can also fall back when other windows are brought into focus.

Check also changing the images. If i do (1) bring the preview window, (2) click on the Image View window, the selected image changes with arrow keys, but the preview image stays the same.

[astaric]

I kind of like it how it work on osx.

Preview opens large automatically adapts it size to the image it displays until I resize it manually. Once manually resized, it will keep the size (and position) until closed.

[ales-erjavec]

The image window now remembers it it was resized by the user and keeps that size when switching between images. Hiding/showing the window will reset this state, reverting back to automatic resize.