update TODO
bus: dbus-daemons' access messages are a bit too verbose
bus: support implicit serialization of strv string arrays within vtables
bus: make it easy to serialize C99 bools and uids/gids/pids via vtable properties
event: add some well-defined priority values of event sources
machined: simplifications
user-sessions: modernizations
localed: mark bus properties as change emitting

Otherwise sd_bus_emit_properties_changed() will refuse sending out
change signals
logind: port logind to libsystemd-bus
logind: fix serialization of ListSeats() bus call