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/msw/textentry.h

@@ -47,6 +47,7 @@ public:
     virtual void SetEditable(bool editable);
 
     virtual void SetMaxLength(unsigned long len);
+    virtual void ForceUpper();
 
 #if wxUSE_UXTHEME
     virtual bool SetHint(const wxString& hint);
@@ -98,5 +99,8 @@ private:
 #endif // wxUSE_OLE
 };
 
+// We don't need the generic version.
+#define wxHAS_NATIVE_TEXT_FORCEUPPER
+
 #endif // _WX_MSW_TEXTENTRY_H_