print: Improve sheet drawing a bit

This is only one step in the right direction. Long term this code should
probably be replaced by real widgets.
print: Center sheet drawing vertically

Center the page ordering indicator vertically, relative to the
checkboxes next to it.
print: Center sheet drawing horizontally

Center the page ordering indicator horizontally, relative to the
spinbutton above it.
printing: Simplify sheet drawing

We don't need the scaling support anymore - icon sizes are
no longer settable.
inspector: Down the model tree

Show nested tree models, inside filter or sort models. Without
this, you can't get at the data.
printing: Another tweak to sheet drawing

After consultation, revisit the vertical centering to center
the first 'stack', and put the second one behind it.
printing: Make GtkPrinter have a useful icon by default

It causes complication elsewhere if gtk_printer_get_icon_name()
returns NULL initially. So make the icon name default to
"printer".
printing: Avoid missing icons

I've noticed a missing image show up in my print dialog, where
the printer reported "printer-paused" as icon. Rearrange things
so we always fall back to "printer" for printer icons.
Updated Brazilian Portuguese translation
Updated German translation