Header: do not let multi-column meters expand to empty neighbors

Extending to right neighbors is intended for text meters with an
overlong content, so the whole text is shown if possible.
Multi column meters, like the combined memory and swap meter, position
its text depending on the given total width; keep the position to the
original assigned header slot.

Short term resolution for #796

MemorySwapMeter.c

@@ -95,6 +95,7 @@ const MeterClass MemorySwapMeter_class = {
    },
    .updateValues = MemorySwapMeter_updateValues,
    .defaultMode = CUSTOM_METERMODE,
+   .isMultiColumn = true,
    .name = "MemorySwap",
    .uiName = "Memory & Swap",
    .description = "Combined memory and swap usage",