Clean up some dead code and MSVC warnings.

- Dead code from experiments in SelectionBar removed.
- Many warnings about unused parameters fixed with WXUNUSED()
- Many warnings about signed / unsigned comparisons cleaned up.
- Several 'local variable declared but not used' warnings fixed.

src/tracks/ui/TrackButtonHandles.cpp

@@ -152,7 +152,7 @@ MenuButtonHandle::~MenuButtonHandle()
 }
 
 UIHandle::Result MenuButtonHandle::CommitChanges
-(const wxMouseEvent &, AudacityProject *pProject, wxWindow *pParent)
+(const wxMouseEvent &, AudacityProject *pProject, wxWindow *WXUNUSED(pParent))
 {
    auto pPanel = pProject->GetTrackPanel();
    auto pCell = mpCell.lock();