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/textentry.h

@@ -138,10 +138,16 @@ public:
     virtual void SetEditable(bool editable) = 0;
 
 
+    // input restrictions
+    // ------------------
+
     // set the max number of characters which may be entered in a single line
     // text control
     virtual void SetMaxLength(unsigned long WXUNUSED(len)) { }
 
+    // convert any lower-case characters to upper-case on the fly in this entry
+    virtual void ForceUpper();
+
 
     // hints
     // -----
@@ -208,6 +214,10 @@ public:
             SuppressTextChangedEvents();
     }
 
+    // change the entry value to be in upper case only, if needed (i.e. if it's
+    // not already the case)
+    void ConvertToUpperCase();
+
 protected:
     // flags for DoSetValue(): common part of SetValue() and ChangeValue() and
     // also used to implement WriteText() in wxMSW