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.

SwapMeter.c

@@ -56,6 +56,7 @@ MeterClass SwapMeter_class = {
    },
    .setValues = SwapMeter_setValues, 
    .defaultMode = BAR_METERMODE,
+   .maxItems = 1,
    .total = 100.0,
    .attributes = SwapMeter_attributes,
    .name = "Swap",