Revised fix for Bug 1977 - Using Spectrogram Settings in TCP causes Audacity to reset Project Rate to default 44100 Hz

This revised fix:
- No longer attempts to drive "Default sample rate" from the selection toolbar.
- No longer drives 'Project rate' direct from prefs.  It's usual it comes from the project.
- Instead 'Project rate' in the project might change if 'Default sample rate' pref is updated, but ONLY if the project has no tracks.
- When 'Project rate' in the project is updated, that is now always signaled to the selection toolbar.  Previously it wasn't.

src/Project.cpp

@@ -1351,7 +1351,16 @@ void AudacityProject::UpdatePrefsVariables()
    gPrefs->Read(wxT("/GUI/TracksFitVerticallyZoomed"), &mTracksFitVerticallyZoomed, false);
    //   gPrefs->Read(wxT("/GUI/UpdateSpectrogram"), &mViewInfo.bUpdateSpectrogram, true);
 
-   gPrefs->Read(wxT("/SamplingRate/DefaultProjectSampleRate"), &mRate, AudioIO::GetOptimalSupportedSampleRate());
+   // The DefaultProjectSample rate is the rate for new projects.
+   // Do not change this project's rate, unless there are no tracks.
+   if( GetTrackCount() == 0){
+      gPrefs->Read(wxT("/SamplingRate/DefaultProjectSampleRate"), &mRate, AudioIO::GetOptimalSupportedSampleRate());
+      // If necessary, we change this rate in the selection toolbar too.
+      auto bar = GetSelectionBar();
+      if( bar ){
+         bar->SetRate( mRate );
+      }
+   }
    mDefaultFormat = QualityPrefs::SampleFormatChoice();
 
    gPrefs->Read(wxT("/AudioIO/SeekShortPeriod"), &mSeekShort, 1.0);
@@ -1559,6 +1568,8 @@ double AudacityProject::AS_GetRate()
    return mRate;
 }
 
+// Typically this came from the SelectionToolbar and does not need to 
+// be communicated back to it.
 void AudacityProject::AS_SetRate(double rate)
 {
    mRate = rate;

src/toolbars/SelectionBar.cpp

@@ -116,6 +116,8 @@ SelectionBar::SelectionBar()
    // will occur.
    // Refer to bug #462 for a scenario where the division-by-zero causes
    // Audacity to fail.
+   // We expect mRate to be set from the project later.
+   // We could just use AudioIO::GetOptimalSupportedSampleRate() here.
    mRate = (double) gPrefs->Read(wxT("/SamplingRate/DefaultProjectSampleRate"),
       AudioIO::GetOptimalSupportedSampleRate());
 
@@ -341,7 +343,12 @@ void SelectionBar::Populate()
 
 void SelectionBar::UpdatePrefs()
 {
-   mRate = (double) gPrefs->Read(wxT("/SamplingRate/DefaultProjectSampleRate"), AudioIO::GetOptimalSupportedSampleRate());
+   // The project rate is no longer driven from here.
+   // IF /SamplingRate/DefaultProjectSampleRate has changed, that is sent to the project.
+   // IF the project has no tracks, then the project updates its sample rate, and
+   // signals that back to the SelectionBar via SetRate().
+   // Usually though there are tracks, and changes to that pref will only affect new
+   // or empty projects.
 
    wxCommandEvent e;
    e.SetInt(mStartTime->GetFormatIndex());
@@ -658,9 +665,6 @@ void SelectionBar::SetRate(double rate)
       // if the rate is actually being changed
       mRate = rate;   // update the stored rate
       mRateBox->SetValue(wxString::Format(wxT("%d"), (int)rate));
-      // Update rate in stored prefs.
-      // This will also update rate in preferences dialog.
-      gPrefs->Write(wxT("/SamplingRate/DefaultProjectSampleRate"), mRate);
 
       // update the TimeTextCtrls if they exist
       NumericTextCtrl ** Ctrls[5] = { &mStartTime, &mEndTime, &mLengthTime, &mCenterTime, &mAudioTime };
@@ -676,7 +680,6 @@ void SelectionBar::OnRate(wxCommandEvent & WXUNUSED(event))
    if (mRateBox->GetValue().ToDouble(&mRate) && // is a numeric value
          (mRate != 0.0))
    {
-      gPrefs->Write(wxT("/SamplingRate/DefaultProjectSampleRate"), mRate);
       NumericTextCtrl ** Ctrls[5] = { &mStartTime, &mEndTime, &mLengthTime, &mCenterTime, &mAudioTime };
       int i;
       for(i=0;i<5;i++)