This patch tracks the total idle time of all workers.
{F2189876}
PS: I have mixed feelings about efficiency. Each second this loops over all units to count the idle units. Looping over all units happens quite often on each tick, e.g. `MiniMapIdleWorkerButton` calls `Engine.GuiInterfaceCall("HasIdleUnits")`, which loops over all units, in each tick. So this operation in itself may be not that expensive, but still...
PPS: A better way would be that idle units have events, like units created or destroyed. This would be more efficient for MiniMapIdleWorkerButton as well as this StatisticsTracker. But this change requires a lot more work.