Added logic, API and docs for wxDataViewModel::HasDefaultCompare indicating a compare function usable without and column (header)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48345 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dataview.h

@@ -156,6 +156,7 @@ public:
     // default compare function
     virtual int Compare( const wxDataViewItem &item1, const wxDataViewItem &item2, 
                          unsigned int column, bool ascending );
+    virtual bool HasDefaultCompare() { return false; }
 
 protected:
     // the user should not delete this class directly: he should use DecRef() instead!
@@ -198,6 +199,7 @@ public:
     
     virtual int Compare( const wxDataViewItem &item1, const wxDataViewItem &item2, 
                          unsigned int column, bool ascending );
+    virtual bool HasDefaultCompare() { return true; }
 
     // implement base methods