Preview resize 12319 (rebased onto develop)



This is the same as gh-2597 but rebased onto develop.

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The Preview viewport should now resize when the right panel is resized.

To test:
 - Browse to an Image and select the "Preview" tab on right.
 - Resize the right panel, using the drag handle.
 - The viewport should resize when you stop the drag (larger and smaller).
 - When a new image is selected and loaded into viewport, should still fit current size.
 - Various combinations of change in panel size, tab selection and image selection should "just work".