Fix wxSearchCtrl::ChangeValue() to actually change value

This was broken because wxSearchCtrl inherited the base class version of
ChangeValue() which didn't really work for it due to the poor way in
which wxTextEntry is designed (see #18071).

Closes #16998.

include/wx/generic/srchctlg.h

@@ -88,6 +88,8 @@ public:
     // operations
     // ----------
 
+    virtual void ChangeValue(const wxString& value) wxOVERRIDE;
+
     // editing
     virtual void Clear() wxOVERRIDE;
     virtual void Replace(long from, long to, const wxString& value) wxOVERRIDE;