Update Play button appearance in idle time...

... so most calls to ControlToolBar::SetPlay are removed.  One remains in
TransportMenus, which will not be problematic for untangling dependencies,
and one remains where the toolbar remakes its own buttons.

But the routines that start and stop the streams, importantly, don't use it.

src/ProjectAudioManager.cpp

@@ -192,6 +192,18 @@ void ProjectAudioManager::OnSoundActivationThreshold()
    }
 }
 
+bool ProjectAudioManager::Playing() const
+{
+   auto gAudioIO = AudioIO::Get();
+   return
+      gAudioIO->IsBusy() &&
+      ControlToolBar::Get( mProject ).CanStopAudioStream() &&
+      // ... and not merely monitoring
+      !gAudioIO->IsMonitoring() &&
+      // ... and not punch-and-roll recording
+      gAudioIO->GetNumCaptureChannels() == 0;
+}
+
 bool ProjectAudioManager::Recording() const
 {
    auto gAudioIO = AudioIO::Get();
@@ -265,7 +277,6 @@ bool DoPlayStopSelect
    //If busy, stop playing, make sure everything is unpaused.
    if (scrubber.HasMark() ||
        gAudioIO->IsStreamActive(token)) {
-      toolbar.SetPlay(false);        //Pops
       toolbar.SetStop(true);         //Pushes stop down
 
       // change the selection
@@ -321,7 +332,6 @@ void DoPlayStopSelect(AudacityProject &project)
       toolbar.OnStop(evt);
    else if (!gAudioIO->IsBusy()) {
       //Otherwise, start playing (assuming audio I/O isn't busy)
-      //toolbar->SetPlay(true); // Not needed as set in PlayPlayRegion()
       toolbar.SetStop(false);
 
       // Will automatically set mLastPlayMode