Fixing bug in opening of files of length 6 chars, reported by P. Williams
HighPassFilterAction: attempt to clarify "revised" behavior

In both comments in the implementation and the description in the GUI, try to
emphasize that in StoreRevised mode, the revised image gets set to the filter
residual, i.e., the difference between the high-pass filtered data and the
original data, i.e., the data as processed with a low-pass filter.
src/gui/strategyframes/frequencyselectionframe.h: add this

This adds a GUI editor for the FrequencySelectionAction. It required adding a
public getter/setter for the "clipDown" parameter to this action.
Merge commit '71e6773c5bb3135ef1b6704dd9342c9d75e00eaf'