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/HitTestResult.h

@@ -0,0 +1,50 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+HitTestResult.h
+
+Paul Licameli
+
+**********************************************************************/
+
+#ifndef __AUDACITY_HIT_TEST_RESULT__
+#define __AUDACITY_HIT_TEST_RESULT__
+
+#include <wx/string.h>
+
+class UIHandle;
+class wxCursor;
+
+struct HitTestPreview
+{
+   HitTestPreview()
+   {}
+
+   HitTestPreview(const wxString &message_, wxCursor *cursor_
+      , unsigned refreshCode_ = 0)
+      : message(message_), cursor(cursor_), refreshCode(refreshCode_)
+   {}
+
+   wxString message {};
+   wxCursor *cursor {};
+
+   // Making this non-zero allows mouse-over highlighting
+   // See RefreshCode.h for bit flags:
+   unsigned refreshCode {};
+};
+
+struct HitTestResult
+{
+   HitTestResult()
+   {}
+
+   HitTestResult(HitTestPreview preview_, UIHandle *handle_)
+      : preview(preview_), handle(handle_)
+   {}
+
+   HitTestPreview preview {};
+   UIHandle *handle {};
+};
+
+#endif