Explicit "maxItems" property of meters

Two changes in this commit:
- All meters now explicitly specify "maxItems" property, even for just
  1 item. (Exception is "container" CPU meter classes, which use
  CUSTOM_METERMODE.)
- "maxItems" being 0 is now allowed. This will let bar meters and graph
  meters render an empty meter.

TasksMeter.c

@@ -74,8 +74,8 @@ MeterClass TasksMeter_class = {
    },
    .setValues = TasksMeter_setValues, 
    .defaultMode = TEXT_METERMODE,
-   .total = 100.0,
    .maxItems = 4,
+   .total = 100.0,
    .attributes = TasksMeter_attributes, 
    .name = "Tasks",
    .uiName = "Task counter",