ListStrEditor `context menu` behaviour probably broken

It looks that the behaviour regarding right_clicking and context_menus is probably broken.
- The are two methods named `_on_context_menu` defined in the file. As a result the `right_click` functionality is discarded (see https://github.com/enthought/traitsui/blob/master/traitsui/qt4/list_str_editor.py#L350 and https://github.com/enthought/traitsui/blob/master/traitsui/qt4/list_str_editor.py#L388).
- It also looks that contrary to the docstring in ListStrEditor about using the default context menu when `menu=None` the `setContextMenuPolicy` is always set to `QtCore.Qt.CustomContextMenu` (see https://github.com/enthought/traitsui/blob/master/traitsui/qt4/list_str_editor.py#L474)