Preview dialog should have better controls when adjusting the scan area

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It is desired to have better control when adjusting the scan area edges/corners. What can help:

• Full-screen mode for the preview dialog
• Zoom in/out in the preview dialog
• Handles in the middle of the edges so you can adjust each side independently
• Automatically zoom in on the corner when corner handles are picked up
• Double-clicking on the edge will snap it to the related edge of the previewed doc