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.

ClockMeter.c

@@ -34,6 +34,7 @@ MeterClass ClockMeter_class = {
    },
    .setValues = ClockMeter_setValues, 
    .defaultMode = TEXT_METERMODE,
+   .maxItems = 1,
    .total = 1440, /* 24*60 */
    .attributes = ClockMeter_attributes,
    .name = "Clock",