Update the directory icon in wxGenericDirCtrl::ExpandDir().

Update the directory icon in wxGenericDirCtrl::ExpandDir().

Calling ExpandDir() to notify the control that a new item was added in a
previously empty directory didn't work as wxTreeCtrl::SetItemHasChildren()
wasn't called.

Fix this by moving SetItemHasChildren() to ExpandDir() from OnExpandItem(), it
must be always done and not just in response to an interactive action.

Closes #12735.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66311 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775