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_hpgl.h

@@ -61,7 +61,7 @@ public:
     /**
      * At the start of the HPGL plot pen speed and number are requested.
      */
-    virtual bool StartPlot() override;
+    virtual bool StartPlot( const wxString& aPageNumber ) override;
 
     /**
      * HPGL end of plot: sort and emit graphics, pen return and release.