Feature: Support pack() for CheckBox/RadioButton/Button

[penguinolog]

Partial #421

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)

[coveralls]

coverage: 74.311% (+0.7%) from 73.566% when pulling ae2926e on penguinolog:feature_wimp_pack into b70be02 on urwid:master.

[wardi]

This is ok, but maybe add a comment in the constructor that the pack method below assumes the wrapped widget won't change.

Possibly better would be to implement pack in Columns so that we don't have to define it here, don't need special warnings in the code and make it useful in more cases.