Add tooltips for state pictograms

[julianbrost]

As far as I can tell, Icinga DB Web provides no textual description of the state indicator and the pictograms included in it:

In comparison, the monitoring module shows a tooltip when hovering the pictograms:

I don't think it's obvious that :zap: is supposed to mean flapping, so I think it would be really helpful to provide a textual description as well.