Don't grab focus when calling wxTreeCtrl::SelectItem().

The workaround for the unexpected events order introduced in r49588 should
only apply to the situation when the user selects an item in the tree, not
when it's done programmatically as this results in unexpected focus changes
(see #11274).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65905 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775