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/carbon/dataview.cpp

@@ -1107,6 +1107,23 @@ void wxMacDataViewDataBrowserListViewControl::SetCurrentItem(const wxDataViewIte
     wxFAIL_MSG( "unimplemented for Carbon" );
 }
 
+int wxMacDataViewDataBrowserListViewControl::GetSelectedItemsCount() const
+{
+  Handle handle(::NewHandle(0));
+
+  if ( GetItems(kDataBrowserNoItem,true,kDataBrowserItemIsSelected,handle) != noErr )
+  {
+      wxFAIL_MSG( "failed to get selected items" );
+      return 0;
+  }
+
+  size_t noOfItems = static_cast<size_t>(::GetHandleSize(handle)/sizeof(DataBrowserItemID));
+  HUnlock(handle);
+  DisposeHandle(handle);
+
+  return noOfItems;
+}
+
 int wxMacDataViewDataBrowserListViewControl::GetSelections(wxDataViewItemArray& sel) const
 {
   size_t noOfSelectedItems;