Improve handing of the enter key in wxComboBox control

* Copy-paste the code for OnChar in the wxTextCtrl.
* This implements two features:
  1. This adds support for the wxTE_PROCESS_ENTER style.
  2. Support for pressing the default button in a dialog if there is one.

include/wx/osx/combobox.h

@@ -131,6 +131,7 @@ protected:
     virtual void EnableTextChangedEvents(bool enable) wxOVERRIDE;
 
     // callbacks
+    void OnChar(wxKeyEvent& event); // Process 'enter' if required
     void OnKeyDown(wxKeyEvent& event); // Process clipboard shortcuts
 
     // the subcontrols