Implement member functions of classes in corresponding .cpp files...

... in four cases; not in some other .cpp file.

This is another move that causes the generated graph to reflect dependencies
correctly.

This fixes other large, hidden cycles that involved PrefsDialog.cpp: there was
link dependency on that when PrefsPanel.h was used for the base class.  No
longer.

Also cycles involving TrackPanel.cpp, which contained the default
implementations for TrackPanelCell and related abstract base classes.

src/TimerRecordDialog.cpp

@@ -85,7 +85,7 @@ enum {
 const int kSlowTimerInterval = 1000; // ms
 
 // This timer interval is used in some busy-wait loops and is much shorter.
-// const int kTimerInterval = 50; // ms
+const int kTimerInterval = 50; // ms
 
 static double wxDateTime_to_AudacityTime(wxDateTime& dateTime)
 {