Make image enhancement configurable.

[hajicj]

The image processing settings should be configurable, with a Settings panel of their own.

• Median filter kernel size
• Background lightness threshold.
• Do background thresholding?

[hajicj]

The median filter is applied during the enhancements in #135 to find a robust estimate of the intensity range of the image. The median filter kernel size is an integer that defines the size of the window over which to compute the median for a given point. Has to be an odd number (because the square has to have a center pixel). Larger means slower and more robust, but beyond 121 is generally useless. A good range for this robustness purpose is 21 - 121.

[hajicj]

The background lightness threshold is applied after the range normalization in #135. It pushes all pixels that are not light enough to the darkest (which is generally 0, after intensity range normalization). "Light enough" means that their intensity is lower than (original lightest - background lightness threshold).