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.

include/wx/osx/textentry.h

@@ -50,6 +50,8 @@ public:
     // in a single line text control
     virtual void SetMaxLength(unsigned long len);
 
+    virtual void ForceUpper();
+
     // writing text inserts it at the current position;
     // appending always inserts it at the end
     virtual void WriteText(const wxString& text);