Use Settings for default sample rate and format...

... The first is an example of a Setting with a computed default value.

Also making a new file to hold them, separate from the GUI that changes them
in QualityPrefs; fewer things depend on QualityPrefs

src/toolbars/SelectionBar.cpp

@@ -60,6 +60,7 @@ with changes in the SelectionBar.
 #include "../ProjectSettings.h"
 #include "../Snap.h"
 #include "../ViewInfo.h"
+#include "../prefs/QualitySettings.h"
 #include "../AllThemeResources.h"
 
 #if wxUSE_ACCESSIBILITY
@@ -123,8 +124,7 @@ SelectionBar::SelectionBar( AudacityProject &project )
    // 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.
-   mRate = (double) gPrefs->Read(wxT("/SamplingRate/DefaultProjectSampleRate"),
-      AudioIOBase::GetOptimalSupportedSampleRate());
+   mRate = (double) QualitySettings::DefaultSampleRate.Read();
 
    // Selection mode of 0 means showing 'start' and 'end' only.
    mSelectionMode = gPrefs->ReadLong(wxT("/SelectionToolbarMode"),  0);