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

include/wx/osx/core/dataview.h

@@ -89,6 +89,7 @@ public:
   virtual wxDataViewItem GetCurrentItem() const = 0;
   virtual void SetCurrentItem(const wxDataViewItem& item) = 0;
 
+  virtual int  GetSelectedItemsCount() const = 0;
   virtual int  GetSelections(wxDataViewItemArray& sel)   const = 0; // returns all selected items in the native control
   virtual bool IsSelected   (wxDataViewItem const& item) const = 0; // checks if the passed item is selected in the native control
   virtual void Select       (wxDataViewItem const& item)       = 0; // selects the passed item in the native control