wxTextValidator improvements

Improve char inclusion/exclusion support; update the sample to show more
features of this class and add a unit test for it.

Closes https://github.com/wxWidgets/wxWidgets/pull/1093

docs/changes.txt

@@ -104,6 +104,10 @@ Changes in behaviour which may result in build errors
   e.g. if the error is due to spelling an option name wrongly, fixing or
   removing its name.
 
+- wxTextValidator::Get{In,Ex}cludes() now return a const reference to
+  wxArrayString. Please update your code to use the appropriate setter
+  Set[Char]{In,Ex}cludes(), instead of mutating the internal data directly.
+
 
 3.1.3: (released 2019-??-??)
 ----------------------------