Move code for handling the menu button out of TrackPanel, but...

... Still to do, move out the code for menu items.  They remain accessible
from Shift-M but not from the menu button.

And ESC key is now implemented.

src/tracks/ui/TrackButtonHandles.cpp

@@ -8,6 +8,7 @@ Paul Licameli split from TrackPanel.cpp
 
 **********************************************************************/
 
+#include "../../Audacity.h"
 #include "TrackButtonHandles.h"
 
 #include "../../HitTestResult.h"
@@ -124,3 +125,44 @@ HitTestResult CloseButtonHandle::HitTest
    else
       return {};
 }
+
+////////////////////////////////////////////////////////////////////////////////
+
+MenuButtonHandle::MenuButtonHandle()
+   : ButtonHandle{ TrackPanel::IsPopping }
+{
+}
+
+MenuButtonHandle::~MenuButtonHandle()
+{
+}
+
+MenuButtonHandle &MenuButtonHandle::Instance()
+{
+   static MenuButtonHandle instance;
+   return instance;
+}
+
+UIHandle::Result MenuButtonHandle::CommitChanges
+(const wxMouseEvent &, AudacityProject *, wxWindow *pParent)
+{
+   return mpCell->DoContextMenu(mRect, pParent, NULL);
+}
+
+HitTestResult MenuButtonHandle::HitTest
+(const wxMouseEvent &event, const wxRect &rect, TrackPanelCell *pCell)
+{
+   wxRect buttonRect;
+   TrackInfo::GetTitleBarRect(rect, buttonRect);
+
+   if (buttonRect.Contains(event.m_x, event.m_y)) {
+      Instance().mpCell = pCell;
+      Instance().mRect = buttonRect;
+      return {
+         HitPreview(),
+         &Instance()
+      };
+   }
+   else
+      return {};
+}