Per-core CPU graphs / Lag compensation

Per-core CPU graphs:

By passing core=0, core=1, etc parameters to the widget.

The default value (core="all") is still to show all cores

Lag compensation:

If the update interval is higher than what it should be, push the same
value several times to the graph to compensate this lag.

Every value shown by the cpu graph is actually an average since the last
lookup, so this actually gives nice average values