Pre-reset 'show' for process.

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.

ProcessList.c

@@ -311,6 +311,7 @@ void ProcessList_scan(ProcessList* this) {
    for (int i = 0; i < Vector_size(this->processes); i++) {
       Process* p = (Process*) Vector_get(this->processes, i);
       p->updated = false;
+      p->show = true;
    }
 
    this->totalTasks = 0;