Pen redistribution tied to Displayed Shapes slider

[HanzPetrov]

Hi Ollie, This tweak ties pen redistribution to the Displayed Shapes slider. Note that the pen redistribution will apply to exports whether the "Apply to export" checkbox is ticked or not, so this behaviour may need to be modified or made optional. Any thoughts welcome. Hanz

[SonarSonic]

I'm going to leave this unmerged for now, until we work out how distribution will be handled in the future!