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

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