Add ISO 269 envelope formats

In the enum QPrinter::PaperSize there aren't the following ISO 269 envelope formats:

*C6 (114 x 162) for A4 folded twice (A6)
*C6/C5 (114 x 229) for A4 folded twice (1/3 A4)
*C5 (162 x 229) for A4 folded once (A5)
*C4 (229 x 324) for A4
*C3 (324 x 458) for A3
*E4 (280 x 400) for B4

This info is here: http://en.wikipedia.org/wiki/Envelope#International_standard_sizes
More info about international standard paper sizes: http://www.cl.cam.ac.uk/~mgk25/iso-paper.html

I don't know how could affect the addition of these sizes to the rest of Qt, but if it's ok and since they are standard sizes and I'm writing a program for printing on envelopes I'd appreciate them to be included.

Thank you in advance for your time