inspector: Fix a thinko

We still need to possibly scan the toplevel after selecting
a widget, since we don't monitor the list of toplevels, so
the window may not be in our tree yet. This was broken in
commit e7bd73f2b0db50c00ef390b756c6597674d7b473.
Remove an unused variable
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.
docs: Add a man page fro gtk3-demo-application

It is an installed binary, so it needs a man page!
print: Add a .paper style class

... and use it in the print preview.
docs: Fix some copy/paste errors
Small code rearrangement
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.
Adwaita: added style for vertically linked entries
widget-factory: More vertical linking examples
widget-factory: Cosmetic improvement
Trivial documentation tweaks
Add a function to load css from a resource

This is common enough that we should just provide an API for it
- we already had a function for this purpose anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=672946
Fix a thinko in the previous commit

GTK+ relies on this function silently failing for invalid data.
docs: Add an index for 3.16 additions