Add wxPrintout::SetUp() to reuse the same code in all ports

MSW, GTK, OSX and Postscript implementations all did almost exactly the
same thing to initialize wxPrintout, so extract this common code into a
new wxPrintout method and just call it instead.

There should be no changes in behaviour.

include/wx/prntbase.h

@@ -281,6 +281,10 @@ public:
 
     virtual wxString GetTitle() const { return m_printoutTitle; }
 
+    // Port-specific code should call this function to initialize this object
+    // with everything it needs, instead of using individual accessors below.
+    bool SetUp(wxDC& dc);
+
     wxDC *GetDC() const { return m_printoutDC; }
     void SetDC(wxDC *dc) { m_printoutDC = dc; }