Add a delay before loading the image to "fast scroll" through several images

[shaperilio]

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[shaperilio]

Probably best to implement this as an action buffer, i.e. maybe the GUI only updates twice per second and all keystrokes within one period are absorbed and applied all at once. This is great for scrolling, level curve changes, etc.

[shaperilio]

For loading, we should consider just reading images in a separate, interruptible thread. We want instantaneous response if the images are small, but interruptible action if scrolling through a bunch of giant images.