Add variable softness to fill tool and contiguous selection tool

This adds a softness parameter to the fill and contiguous selection tools.

• Adds the functionality to KisScanlineFill/KisFillPainter/FillProcessingVisitor
• Now the smooth/not-smooth variants are separated in two selection policies in KisScanlineFill
• Changed implementation of the selection policies to use only integers
• Add slider in the fill tool to set the softness
• Add slider in the contiguous fill tool to set the softness and hide the antialias option
• Now the contiguous fill tool uses the flood fill policies for softness so no custom code to make the selection aliased is needed.

NOTE: Some tests fail. I think that may be because of one or two things:

• I changed the selection policies to use only integer arithmetic. (It seems to be the next point).
• I changed some conditions in the hard variants of those policies. In HardSelectionPolicy::calculateOpacity and HardSelectionPolicyExtended::calculateOpacity member functions (the not-smooth variants) I use:

  return this->calculateDifference(pixelPtr) < m_threshold && this->calculateSelectedness(x, y) > 0 ? MAX_SELECTED : MIN_SELECTED;

  instead of the previous:

  if (!useSmoothSelection) {
      return diff <= m_threshold ? MAX_SELECTED : MIN_SELECTED;
  }

  The decision to change that is because I think there was a difference between the smooth and not-smooth code. The smooth code was:

  quint8 selectionValue = qMax(0, m_threshold - diff);
  quint8 result = MIN_SELECTED;
  if (selectionValue > 0) {
      qreal selectionNorm = qreal(selectionValue) / m_threshold;
      result = MAX_SELECTED * selectionNorm;
  }
  result;

  In this case if the difference is equal to the threshold, the code in the if is not executed and the result is MIN_SELECTED, the opposite as in the not-smooth code, where if diff <= m_threshold the result is MAX_SELECTED.
  I dont know If I should leve the code as before or as I changed it (or maybe change the smooth case intead of the not-smooth one). Also, I guess that if it is changed I should fix the tests?

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