Define and use the UIHandle and TrackPanelCell protocols, but...

...no actions reimplemented to them yet.

Later commits will move special cases one at a time from TrackPanel, preserving
all click and drag capabilities at each step.  With a few exceptions, but those
lost abilities are restored in yet later commits.  (Ctrl+Click on the Label
track being one.)

src/UIHandle.cpp

@@ -0,0 +1,29 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+UIHandle.cpp
+
+Paul Licameli
+
+**********************************************************************/
+
+#include "UIHandle.h"
+
+UIHandle::~UIHandle()
+{
+}
+
+void UIHandle::DrawExtras
+   (DrawingPass, wxDC *, const wxRegion &, const wxRect &)
+{
+}
+
+bool UIHandle::StopsOnKeystroke()
+{
+   return false;
+}
+
+void UIHandle::OnProjectChange(AudacityProject *)
+{
+}