wxDataViewCtrl::GetSelection() now always returns invalid item if more than a single item is selected in a multi-selection control. Also add HasSelection() and GetSelectedItemsCount() to allow checking if any items are selected. Updated the documentation, all ports and added a test for all these functions. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68844 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775