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/osx/cocoa/dataview.mm

@@ -2265,6 +2265,11 @@ void wxCocoaDataViewControl::SetCurrentItem(const wxDataViewItem& item)
     Select(item);
 }
 
+int wxCocoaDataViewControl::GetSelectedItemsCount() const
+{
+    return [m_OutlineView numberOfSelectedRows];
+}
+
 int wxCocoaDataViewControl::GetSelections(wxDataViewItemArray& sel) const
 {
     NSIndexSet* selectedRowIndexes([m_OutlineView selectedRowIndexes]);