gdktoplevel-wayland: Test for both versions of zxdg_importer

Some compositors, such as KWin, only support the zxdg_importer_v2 protocol. The test in gdk_wayland_toplevel_set_transient_for_exported only checks for the v1 protocol, even though both are supported. This meant that portal file dialogs failed to appear as modal while using KDE Plasma.