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.

interface/wx/textentry.h

@@ -212,6 +212,17 @@ public:
     */
     virtual void Cut();
 
+    /**
+        Convert all text entered into the control to upper case.
+
+        Call this method to ensure that all text entered into the control is
+        converted on the fly to upper case. If the control is not empty, its
+        existing contents is also converted to upper case.
+
+        @since 3.1.0
+     */
+    void ForceUpper();
+
     /**
         Returns the insertion point, or cursor, position.