gtkwindow: Do not create the multipress gesture for plugs

GtkPlugs may "qualify" as toplevels, even though they're not meant to
be WM manipulated, so refuse to create the multipress gesture for these
too.
gtkwindow: Resort to regular event bubbling by all means on non-toplevels

If the multipress gesture is not created (ie. not a toplevel GtkWindow),
also avoid possibly calling gtk_widget_event() on the events gotten here.