Replace wxPrintout::SetIsPreview() with SetPreview().

Unlike the old function just indicating whether a printout is being used for
previewing, the new one associates the preview object with it. This can be
useful if we need to access the window used for the preview, for example.

Also remove a bunch of apparently unnecessary SetIsPreview(false) calls as
printing (and not previewing) is already the default.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64212 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/print.h

@@ -708,9 +708,26 @@ public:
 
     /**
         Returns @true if the printout is currently being used for previewing.
+
+        @see GetPreview()
     */
     virtual bool IsPreview() const;
 
+    /**
+        Returns the associated preview object if any.
+
+        If this printout object is used for previewing, returns the associated
+        wxPrintPreview. Otherwise returns @NULL.
+
+        The returned pointer is not owned by the printout and must not be
+        deleted.
+
+        @see IsPreview()
+
+        @since 2.9.1.
+     */
+    wxPrintPreview *GetPreview() const;
+
     /**
         Set the user scale and device origin of the wxDC associated with this wxPrintout
         so that one screen pixel maps to one device pixel on the DC.