KpiSparkLine: bridge gaps with a dashed, hatched line

A gap in the series just broke the curve, with nothing marking it as
missing. Bridge it instead: a dashed line between the two real
points, hatched underneath.

Also add a "Data state" playground control (Complete/Gap) producing
one contiguous outage window, replacing the old "gap every N points"
knob.

CMK-38083

Change-Id: I72b7472c2f3978047d6e635e95c936b2b57fd357