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/dataview_osx.cpp

@@ -535,15 +535,9 @@ wxRect wxDataViewCtrl::GetItemRect(wxDataViewItem const& item, wxDataViewColumn
     return wxRect();
 }
 
-wxDataViewItem wxDataViewCtrl::GetSelection() const
+int wxDataViewCtrl::GetSelectedItemsCount() const
 {
-  wxDataViewItemArray itemIDs;
-
-
-  if (GetDataViewPeer()->GetSelections(itemIDs) > 0)
-    return itemIDs[0];
-  else
-    return wxDataViewItem();
+  return GetDataViewPeer()->GetSelectedItemsCount();
 }
 
 int wxDataViewCtrl::GetSelections(wxDataViewItemArray& sel) const