Say "Connecting…" only when connecting is slow

[Lakr233]

Reopening the app a few seconds after closing it flashed "Connecting to the
inspector service…" for a few hundred milliseconds: long enough to see, too
short to read. The spinner went up the moment the phase turned to
.connecting, however quickly the daemon answered.

DelayedLoadingIndicator keeps the time instead. A wait that ends inside
three quarters of a second shows a blank list and then the rows; a wait that
runs longer shows the indicator, which then stays at least half a second so
it doesn't blink off, and rows that land in that half second are drawn when
it ends. A failure or an empty result never waits: it replaces the
indicator, or the blank, at once. The wait for the first sample after
connecting counts too, so a slow connection no longer drops from the
spinner to an empty list before the rows arrive.

The detail tables had the same flash on every push while their records
loaded, and use the same indicator. Because the indicator now turns up on a
screen that is already showing, VoiceOver is told the layout changed.

The first sample itself is not delayed: firstSampleDelay is the gap
before the second one, which is what CPU use needs. make harness now
steps the indicator through its grace period, minimum time, failure and
retry on macOS.

Fixes #7

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