Add options to edit automatic contrast / levels curves

[dchristensen]

The automatic contrast and automatic levels features are great, but sometimes one needs to make further changes to the curves. It would be nice if one could directly edit the curves produced by the automatic process. A suggested UI for this would be that in the Adjust > Curves tool, one could have buttons "Automatic contrast" and "Automatic Levels" that produce the curves that those tools would use. Then one could adjust those curves before closing the Curves tool.

Because of the low internal precision used by XnView, applying multiple operations results in loss of information (see https://newsgroup.xnview.com/viewtopic.php?p=208529), so it's not ideal to first run one of the automatic tools, and then open the Curves tool and make further changes, especially since the automatic tools sometimes compress shadows and/or highlights, which are then not recoverable.