Make non-error notifications transient

[Layerex]

Summary

Make non-error notifications transient, so those don't show up in notification tray.

How to send a transient notification:

call := obj.Call("org.freedesktop.Notifications.Notify", 0, "gopass", uint32(0), iconURI(), subj, msg, []string{}, map[string]dbus.Variant{"transient": dbus.MakeVariant(true)}, int32(3000))

Also, I think, for transient nofiications exposition time should be lowered from 5 seconds to 3 (standart transient notifications in GNOME seem to use this value).

[dominikschulz]

I think we can easily do that. Thank you.