Issues when Task menu-bar is None

When swapping between Tasks in a TaskWindow, if the new Task has no menu bar then the `menu_bar_manager` of the `TaskWindow` is `None` and as a result the change handlers on the parent `ApplicationWindow` do nothing, leaving the menu bar for the previous task in place.

See https://github.com/enthought/pyface/blob/42fbaf6e18883b8e5a662fc9ba108fd2408c91d3/pyface/ui/qt4/application_window.py#L171-L174
and
https://github.com/enthought/pyface/blob/42fbaf6e18883b8e5a662fc9ba108fd2408c91d3/pyface/ui/qt4/application_window.py#L78-L81

We should probably do something to ensure that when the `menu_bar_manager` changes to `None` that the current menu bar, if any, is removed. It may also be the case that we should ensure that a Task always has at least a minimal menu bar.

This may also affect Toolbars - this hasn't been investigated.

The work around for this is to ensure that end-user `Task` instances always have a menu bar.