Review: zoom implementation and image correction.

[armm77]

• Implementation of zoom in and zoom out via keyboard.

Bug fixes

• Fixes image shifting to coordinate 0,0 when applying zoom instead of staying centered. imageView now always occupies at least the visible scroll area size, allowing NSImageAlignCenter to center the image correctly.

New behaviors

• Image stays centered after every zoom change and when resizing the window.
• Images that fit on screen open at 100% with the window sized to match.
• Images larger than the screen are proportionally scaled on open.
• The popup displays the exact percentage when scaling. If it does not match a fixed value, a temporary dynamic item is added and removed when another zoom level is selected.
• Zoom In and Zoom Out navigate to the next/previous fixed value relative to the current percentage, even when the current zoom is a dynamic value.
• Resizing the window proportionally rescales the image and updates the popup.
• Minimum window size reduced from 200×200 to 100×100.
• Maximum window size limited to the visible screen area (visibleFrame).

[trunkmaster]

Excellent, thanks!