Eeschema: Plot dialogs: unify 5 dialogs (which means 5 duplicate code) to only one plot dialog (like in Pcbnew)
remove duplicate code and clean it.
Simplify HPGL dialog: plot offet removed and replaced by only one option:
plot origin to center of sheet or not (some HPGL  plotters have the plot origin to center of the paper area,
and others have this origin at the bottom left corner)
This is surely enough.