examples/terminal.py can run against older versions of Urwid again

[danschwarz]

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:

terminal.py now enables bracketed paste mode and shows columns/rows when run against the latest Urwid library which supports those features. When run against older versions, it will now disable those features rather than throw an exception.

Also, improved documentation for raw_display.Screen. I am not sure how to test that the new documentation is working correctly, as it doesn't generate the HTML pages on my local system. The changes should be visible on the new version of this page

[coveralls]

coverage: 73.458%. remained the same when pulling f372c89 on danschwarz:smallfix into a8c0d71 on urwid:master.