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.

CPUMeter.c

@@ -312,8 +312,8 @@ MeterClass LeftCPUs2Meter_class = {
    .total = 100.0,
    .attributes = CPUMeter_attributes, 
    .name = "LeftCPUs2",
-   .description = "CPUs (1&2/4): first half in 2 shorter columns",
    .uiName = "CPUs (1&2/4)",
+   .description = "CPUs (1&2/4): first half in 2 shorter columns",
    .caption = "CPU",
    .draw = DualColCPUsMeter_draw,
    .init = AllCPUsMeter_init,