This patch implements width tuning (width correction) for PS plotting of tracks, pads and vias.

This feature is very useful for electronics hobbyists who use DIY PCB technology 
(both toner transfer and photoresist methods).
Also width correction may be useful for PCB designers who take care of track width etching.
This patch also fixes some minor PS plotting issues.

pcbnew/pcb_plot_params.h

@@ -86,6 +86,8 @@ public:
     // Only X and Y dimensions are adjusted: circles are plotted as circle, even if X and Y fine scale differ.
     double      m_FineScaleAdjustX;     // fine scale adjust X axis
     double      m_FineScaleAdjustY;     // dine scale adjust Y axis
+    // These width factor is intended to compensate plotters (and mainly printers) line width error.
+    double      m_FineWidthAdjust;
 
 private:
     long        layerSelection;