Move hypertext linking to user-page-numbers.

Also moves most navigation code to SCH_NAVIGATION_TOOL.
Also changes page number href to anchor syntax ('#foo').
Also adds hypertext processing to SCH_TEXTBOXes.
Also adds combobox with schematic pages to text properties dialog.

include/plotters/plotter_gerber.h

@@ -49,7 +49,7 @@ public:
     /**
      * Write GERBER header to file initialize global variable g_Plot_PlotOutputFile.
      */
-    virtual bool StartPlot() override;
+    virtual bool StartPlot( const wxString& pageNumber ) override;
     virtual bool EndPlot() override;
     virtual void SetCurrentLineWidth( int aLineWidth, void* aData = nullptr ) override;