Allow configuring showing printing dialog in wxHtmlEasyPrinting

Add wxHtmlEasyPrinting::SetPromptMode() to allow suppressing the
"prompt" shown by wxPrinter::Print() when it's called from this class
code.

Closes https://github.com/wxWidgets/wxWidgets/pull/838

include/wx/html/htmprint.h

@@ -273,6 +273,16 @@ public:
     void SetName(const wxString& name) { m_Name = name; }
             // set the printout name
 
+    // Controls showing the dialog when printing: by default, always shown.
+    enum PromptMode
+    {
+        Prompt_Never,
+        Prompt_Once,
+        Prompt_Always
+    };
+
+    void SetPromptMode(PromptMode promptMode) { m_promptMode = promptMode; }
+
 protected:
     virtual wxHtmlPrintout *CreatePrintout();
     virtual bool DoPreview(wxHtmlPrintout *printout1, wxHtmlPrintout *printout2);
@@ -296,6 +306,8 @@ private:
     wxString m_Headers[2], m_Footers[2];
     wxWindow *m_ParentWindow;
 
+    PromptMode m_promptMode;
+
     wxDECLARE_NO_COPY_CLASS(wxHtmlEasyPrinting);
 };