printers: Resize the "Add Printer" dialog

In order to keep the dialog size in the same aspect ratio of the
mockups as https://wiki.gnome.org/Design/SystemSettings/Printers

The "height-request" property should never go higher than 490px.

490 is an estimated value for the panels to properly fit on netbook
screens. See https://wiki.gnome.org/Design/SystemSettings#Notes

https://bugzilla.gnome.org/show_bug.cgi?id=778266