Add "read" and "written" fields to the dashboard swap group, which
report the total amount of data read-from/written-to the tile swap,
respetively.  These fields are non-active by default.  When these
fields are active, show a color underlay in the swap group's meter,
indicating when data was beging read-from/written-to the swap.

Improve the swap busy indicator (used as the meter's LED), so that
it's active whenever data has been read-from/written-to the swap
during the last sampling interval, rather than at the point of
sampling.