i3bar: handle ConfigureRequests for tray children (fixes gtk3 size issue)

i3bar: handle ConfigureRequests for tray children (fixes gtk3 size issue)

Thanks to yvesf for this simple python test script:

    from gi.repository import Gtk as gtk

    def cb(*a):
        print a

    def si_popup(*a):
        print a

    status_icon = gtk.StatusIcon()
    status_icon.set_from_stock(gtk.STOCK_OPEN)
    status_icon.connect("activate", cb)

    gtk.main()