Port to GTK4

[danirabbit]

Fixes #8

[danirabbit]

@ryonakano think I fixed this how it's supposed to be. We set hide_on_close property based on the status change of transfer and then call close instead of destroy

[ryonakano]

Is this expected that this destroy () is just removed instead of being replaced by close ()?

I expected this:

        response.connect ((response_id) => {
            if (response_id == Gtk.ResponseType.CANCEL) {
                if (transfer != null) {
                    if (transfer.status == "active") {
                        try {
                            transfer.cancel ();
                        } catch (Error e) {
                            GLib.warning (e.message);
                        }
                        remove_session.begin ();
                    }
                }
            }

            close ();
        });

[ryonakano]

SenderDialog and ReceiverDialog now calls close () instead of destroy () but is destroy () signal still emitted? Doesn't we need to modify this?

[ryonakano]

I found an issue that ScanDialog never closes even if I click Close button, which only happens when the dialog has been once opened and then closed.