Added better visual feedback for tree control

triangles. They now have a sort of prelight
    status like they have under GTK2.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@25836 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/treectlg.h

@@ -423,6 +423,7 @@ protected:
     wxGenericTreeItem   *m_dropTarget;
     wxCursor             m_oldCursor;  // cursor is changed while dragging
     wxGenericTreeItem   *m_oldSelection;
+    wxGenericTreeItem   *m_underMouse; // for visual effects
     wxTreeTextCtrl      *m_textCtrl;
 
     wxTimer             *m_renameTimer;