Reconnect the 'modified' signal after setting a new ListBox.body

[exquo]

Ensure that the cached canvas is marked invalidated when a newly set ListBox.body is modified at runtime.

Fixes #428

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Currently, ListBox listens to modified events from the ListWalker set as its body only on initialization. If a new ListWalker is assigned to ListBox.body later, the signals from it would be ignored.

[penguinolog]

a bit more paranoid validation can be used, but current is already validating issue fix

class ListBoxSetBodyTest(unittest.TestCase):
    def test_signal_connected(self):
        lb = urwid.ListBox([])
        modified_callbacks = lb.body._urwid_signals['modified']
        self.assertEqual(1, len(modified_callbacks))
        callback = modified_callbacks[0][1]
        self.assertEqual(lb._invalidate, callback, "Initial callback was not assigned")

        lb.body = urwid.SimpleListWalker([])
        new_modified_callbacks = lb.body._urwid_signals['modified']
        self.assertEqual(1, len(new_modified_callbacks), "Unexpected amount of callbacks for new body")
        new_callback = new_modified_callbacks[0][1]
        self.assertEqual(lb._invalidate, new_callback, "Callback was not updated")

        self.assertEqual(callback, new_callback)

[exquo]

🎉