printers: Move options dialog spinner out of the action area

For an infinitesimal amount of time after opening the options dialog, a
spinner is displayed at the start of the action area while the dialog
loads asynchronously. Display the spinner in the center of the dialog
instead, using a GtkStack to switch between spinner mode and normal
mode. Test by removing the calls to printer_get_ppd_async(),
get_named_dest_async(), and get_ipp_attributes_async() from the bottom
of populate_options().

Why? (1) It looks better this way. (2) Need to stop using the action
area in order to switch to a header bar.

https://bugzilla.gnome.org/show_bug.cgi?id=755713

https://bugzilla.gnome.org/show_bug.cgi?id=779708