Harmonize wxDataViewCtrl::GetSelection() behaviour in all ports.

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

src/common/datavcmn.cpp

@@ -1094,6 +1094,16 @@ void wxDataViewCtrlBase::SetCurrentItem(const wxDataViewItem& item)
         Select(item);
 }
 
+wxDataViewItem wxDataViewCtrlBase::GetSelection() const
+{
+    if ( GetSelectedItemsCount() != 1 )
+        return wxDataViewItem();
+
+    wxDataViewItemArray selections;
+    GetSelections(selections);
+    return selections[0];
+}
+
 wxDataViewColumn *
 wxDataViewCtrlBase::AppendTextColumn( const wxString &label, unsigned int model_column,
                             wxDataViewCellMode mode, int width, wxAlignment align, int flags )