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This simplifies the protocol between the platform-independent
and platform-specific parts. The platform-specific parts
were supposed to re-determine the value of process->show
on each iteration, and the Darwin subsystem wasn't doing that.
Instead of adding the code to the Darwin part, I lifted the
burden of the OS-specific of resetting process->show: now
they can choose to hide a process if they want to (e.g.
detecting kernel threads) but are not required to
(e.g. on Darwin where we're not listing threads separately (yet?)).
Fixes tree view collapsing/expanding on OSX. Closes #416.