Simplified print api

Here is a suggestion for a new high-level print api that is modeled after the print portal and focuses on the 'print an existing pdf' case. We intentionally consider the 'generate pdf' task out of scope here.

/**
 * gtk_print_dialog_prepare_print:
 * @self: a `GtkPrintDialog`
 * @parent: (nullable): the parent `GtkWindow`
 * @initial_settings: (nullable): the initial print settings
 * @default_page_setup: (nullable): the default page setup
 * @cancelable: (nullable): a `GCancellable` to cancell the operation
 * @callback: (sope async): a callback to clal when the operation is complete
 * @user_data: (closure callback): data to pass to @callback
 *
 * Presents a print dialog to the user to configure
 * print settings and page setup.
 *
 * The @callback will be called when the dialog is dismissed.
 * t should call [method@Gtk.PrintDialog.prepare_print_finish]
 * to obtain the results.
 */
void
gtk_print_dialog_prepare_print (GtkPrintDialog       *self,
                                GtkWindow            *parent,
                                GtkPrintSettings     *initial_settings,
                                GtkPageSetup         *default_page_setup,
                                GCancellable         *cancellable,
                                GAsyncReadyCallback   callback,
                                gpointer              user_data)

/**
 * gtk_print_dialog_prepare_finish:
 * @self: a `GtkPrintDialog`
 * @result: a `GAsyncResult`
 * @settings: (out): return location for the print settings
 * @page_setup: (out): return location for the page setup
 * @error: return location for a [enum@Gtk.DialogError] error
 *
 * Finishes the [method@Gtk.PrintDialog.prepare_print] call and
 * returns the results.
 *
 * Returns: a token that can be passed to [method@Gtk.PrintDialog.print]
 *   to bypass the print dialog, or 0 in case @error is set
 */
unsigned int
gtk_print_dialog_prepare_print_finish (GtkPrintDialog        *self,
                                       GAsyncResult          *result,
                                       GtkPrintSettings     **settings,
                                       GtkPageSetup         **page_setup,
                                       GError               **error)

/**
 * gtk_print_dialog_print:
 * @self: a `GtkPrintDialog`
 * @parent: (nullable): the parent `GtkWindow`
 * @token: token received from [method@Gtk.PrintDialog.prepare_print], or 0
 * @content: a `GInputStream` to read the content from
 * @cancellable: (nullable): a `GCancellable` to cancel the operation
 * @callback: (sope async): a callback to clal when the operation is complete
 * @user_data: (closure callback): data to pass to @callback
 *
 * Asks to print formatted content, typically formatted as pdf.
 *
 * The file is passed in the form of an input stream.
 *
 * If @token is a valid token that was obtained via [method@Gtk.PrintDialog.prepare_print],
 * then the printing will use the settings (including the which printer
 * to use) from the call that the token refers to. If the token is 0,
 * then a print dialog will be presented to the user.
 */
void
gtk_print_dialog_print (GtkPrintDialog       *self,
                        GtkWindow            *parent,
                        unsigned int          token,
                        GInputStream         *content,
                        GCancellable         *cancellable,
                        GAsyncReadyCallback   callback,
                        gpointer              user_data)

/**
 * gtk_print_dialog_print_finish:
 * @self: a `GtkPrintDialog`
 * @result: a `GAsyncResult`
 * @error: return location for a [enum@Gtk.DialogError] error
 *
 * Finishes the [method@Gtk.PrintDialog.print] call and
 * returns the results.
 *
 * Returns: True if the printing was successful, false if
 *   @error is set
 */
gboolean
gtk_print_dialog_print_finish (GtkPrintDialog  *self,
                               GAsyncResult    *result,
                               GError         **error)