Class CheckListCtrlMixin

This is a mixin for ListCtrl which add a checkbox in the first
column of each row. It is inspired by limodou's CheckList.py(which
can be got from his NewEdit) and improved:    
    - You can just use InsertStringItem() to insert new items;
    - Once a checkbox is checked/unchecked, the corresponding item is not
      selected;
    - You can use SetItemData() and GetItemData();
    - Interfaces are changed to OnCheckItem(), IsChecked(), CheckItem().

You should not set a imagelist for the ListCtrl once this mixin is used.