Add wxTreeCtrl::{Clear,Set}FocusedItem().

Allow changing just the currently focused (not selected) item and also
removing the focus completely.

Closes #11599.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64196 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/treectrl.cpp

@@ -2137,6 +2137,8 @@ void wxTreeCtrl::ClearFocusedItem()
 
 void wxTreeCtrl::SetFocusedItem(const wxTreeItemId& item)
 {
+    wxCHECK_RET( item.IsOk(), wxT("invalid tree item") );
+
     TempSetter set(m_changingSelection);
 
     ::SetFocus(GetHwnd(), HITEM(item));