Do not display labels for single data sets in perfdata piecharts

Display generic chart titles for the whole piechart that only contain title and percentage and don't style perfdata piecharts using HTML properties.

refs #7077
refs #6200
refs #7304
Streamline chart implementation

Define sparkline layout through CSS class and remove label/tooltip formatting.

fixes #7077
Rework behavior of perfdata visualization

Determine perfdata pie color from host or service state, display zero percent piecharts, only render displayabl PieCharts with min and max values, move perfdata to piechart conversion functions into the Perfdata object.

fixes #6423
fixes #6200
fixes #7170
fixes #7304
Use neutral default color in perfdata piecharts

If nothing is known about the state of the monitoring object, the piecharts should neither indicate OK nor Critical.