wxHtmlWindow doesn't render HTML text when used only inside our .kiface dll. (Windows and Linux)

* When a wxHtmlWindow is used *only* in a dll/so module, the Html text is displayed
* as plain text.
* this patch forces wxHtmlWinParser initialization to avoid this issue
*see https://groups.google.com/forum/#!topic/wx-users/FF0zv5qGAT0

Fixes: lp:1783544
https://bugs.launchpad.net/kicad/+bug/1783544

common/single_top.cpp

@@ -40,6 +40,7 @@
 #include <wx/filename.h>
 #include <wx/stdpaths.h>
 #include <wx/snglinst.h>
+#include <wx/html/htmlwin.h>
 
 #include <kiway.h>
 #include <pgm_base.h>
@@ -99,6 +100,21 @@ PGM_BASE& Pgm()
     return program;
 }
 
+// A module to allow Html modules initialization/cleanup
+// When a wxHtmlWindow is used *only* in a dll/so module, the Html text is displayed
+// as plain text.
+// This helper class is just used to force wxHtmlWinParser initialization
+// see https://groups.google.com/forum/#!topic/wx-users/FF0zv5qGAT0
+class HtmlModule: public wxModule
+{
+public:
+    HtmlModule() { }
+    virtual bool OnInit() override { AddDependency( CLASSINFO( wxHtmlWinParser ) ); return true; };
+    virtual void OnExit() override {};
+private:
+    wxDECLARE_DYNAMIC_CLASS( HtmlModule );
+};
+wxIMPLEMENT_DYNAMIC_CLASS(HtmlModule, wxModule);
 
 /**
  * Struct APP_SINGLE_TOP
@@ -123,6 +139,11 @@ struct APP_SINGLE_TOP : public wxApp
 
     bool OnInit() override
     {
+        // Force wxHtmlWinParser initialization when a wxHtmlWindow is used only
+        // in a shared modules (.so or .dll file)
+        // Otherwise the Html text is displayed as plain text.
+        HtmlModule html_init;
+
         try
         {
             return program.OnPgmInit();