Doc: add missing GtkPrintSettings symbols

The symbols are documented, they were just missing in gtk3-sections.txt.

https://bugzilla.gnome.org/show_bug.cgi?id=711354
demo: Make sure to add gtk3-widget-factory.desktop to tarball

In addition to 721cdfd966c41b8fb57f77e9c0e409c6a7a98b03
broadway: Extract buffer decoding to separate helper
broadway: Add buffer decoding colorizing

If you add ?debug=decoding to the url you will get colorized debug
info for the buffer decoder.