GUI: Image preview panels doesn't remember zoom settings

[DrMcCoy]

When changing the zoom settings for an image in the image preview panel, and then clicking on a different image in the tree view, the new image will be displayed at 100%, instead of at the zoom level of the previous image.

Instead, the preview panel should remember the settings and apply that to the new images. Moreover, when "Fit" was pressed, the new image should fit inside the panel, even when the size of the new image is different from the size of the old image. I.e. it's not enough to remember the scaling factor, but also the scaling method/operation.

Additionally, when the image is scaled to fit the window, the scaling factor needs to automatically change when the preview panel is resized (either directly, or by resizing the main window), so that the image still correctly fits inside the panel after the size change.