Add wxTextEntry::ForceUpper()

Allow automatically converting lower-case letters entered into wxTextCtrl to
upper-case equivalents. Provide generic fallback and implement the method
natively for all the major platforms.

Also update the text sample to show it in action.

docs/changes.txt

@@ -90,6 +90,7 @@ All (GUI):
 - Allow customizing window shown by wxBusyInfo.
 - Add wxAddRemoveCtrl.
 - Add wxAppProgressIndicator for MSW (Chaobin Zhang) and OS X (Tobias Taschner).
+- Add wxTextEntry::ForceUpper().
 - Add wxEVT_MAGNIFY mouse event (Joost Nieuwenhuijse).
 - Add wxProcess::Activate().
 - Fix setting colours of labels in wxSlider.