Print: Match document size against known paper sizes [v2]
Custom paper sizes work with some printer drivers but not others. EPSON's ESC/P-R CUPS driver for example scales a custom-sized document to ISO A4. (It apparently used to just bail on unknown page sizes.) Matching the document size against a known paper size in Inkscape prevents the driver from scaling to the wrong paper size.
Fixes an issue discussed in comments 3, 4, 5, and 6 of https://bugs.launchpad.net/inkscape/+bug/630635.
Changes from v1:
Inkscape::UI::Dialog::Printfirst, as partly requested by @marcjeanmougin.
Note: This is my first time using gtkmm instead of plain GLib/GTK+, so let me know if anything is wrong with the conversion.