Perfdata plugin should also have a knowledge about labels

Prior to this change the PerfdataSet only knew the labels of a performance
data value which prevented the Perfdata object from being used
individually.

refs #6515
Do not show pie charts for perfdata values when no percentage is available

refs #6515
Improve piechart limitation and show non-piechart perfdata as well

The perfdata helper did an improper limitation as it might have skipped
valid values due to applying the limit before the filter. When not in compact
view the helper now also shows non-piechart values by using their raw
representation.

refs #6515
Merge branch 'bugfix/show-non-piechart-perfdata-not-as-piechart-6515'

fixes #6515