smaps plugin: instants track, datagrid details panel (#6184)

For traces with smaps packets, we now show process-scoped tracks with
instants representing those smaps snapshots. When selected, the list of
mappings is shown in the details panel as a datagrid.

One complication is that typically a trace won't record all possible
value columns (such as rss, anon, etc), so I wanted to render only the
relevant columns in the details panel to make it less
cluttered/misleading. The underlying SQL table doesn't have nullability
(unset readings default to 0), and I am not convinced that adding
nullability is worth the increased effort to query the table by people.

What I've done instead: the entire smaps table is scanned and only
columns that have a nonzero reading are shown in all smaps details
panels for that trace. Similarly, the "aggregate_count" is only shown if
at least one snapshot has aggregation. So there is some ambiguity and a
particular corner case: if a value was recorded, but was truly zero for
all snapshots in the trace (right now that column would end up hidden).

Also, I'm deliberately not doing anything special for a hypothetical
trace with smaps packets recorded with different configs.