Homing: improve homing and feedback on user interface

When homing an axis I would like the user interface to report what is going on better. Specifically:
- is an axis currently homed (and if possible when that was)
- unhomed axes show in alarms view?
- when did the controller last power cycle (which will also affect homing, unless absolute encoders)
- did the home i had just tried to home complete successfully or fail (time out)
- limit lights come on as expected during home (see #5868)
- the home icon meaning is clarified. It may mean "at home" but on some axis it is on all the time and others never on except briefly when it passes a home switch
- fix issue with multiple same galil axes homing at once, or stop such things being requested from the gui. These can lead to a homing timeout, so it may be enough to just clearly flag failed homes as above.