Make wxWS_EX_VALIDATE_RECURSIVELY default (and only) behaviour

In practice, almost everybody using validators also seems to use this style,
so make it the default (this hadn't been done when it was originally
introduced because of compatibility concerns, but now, 15+ years later, it's
probably safe enough to change this).

docs/changes.txt

@@ -27,6 +27,10 @@ Changes in behaviour not resulting in compilation errors
 - Using invalid flags with wxBoxSizer or wxGridSizer items now triggers asserts
   when done from the code or error messages when done in XRC.
 
+- wxWS_EX_VALIDATE_RECURSIVELY is now the default behaviour, i.e. calling
+  Validate() or TransferData{From,To}Window() will now also call the same
+  function for all children.
+
 - wxOSX/Carbon port doesn't exist any more, wxOSX/Cocoa will be silently used
   instead even if configure --with-osx_carbon option is used.