gtkprintbackendcups: Fix a crash in avahi_create_browsers()

gtkprintbackendcups: Fix a crash in avahi_create_browsers()

In avahi_request_printer_list() a new connection to the DBus system bus
is started asynchronously, but it's not cancellable and it's not taking
any reference of the GtkPrintBackendCups. This means that when the
callback is called, the object might have been destroyed already. We can
just pass the cancellable created and check for the cancelled error in
the callback before trying to use the GtkPrintBackendCups. The code to
cancel avahi operations and to unsibscribe from the DBus signals has
been moved from finalize to dispose to make sure it happens as soon as
possible.

https://bugzilla.gnome.org/show_bug.cgi?id=696553