Make selection container titles a tuple of strings

[jasongrout]

get_title and set_title have long been somewhat a wart in our API, requiring people to put several statements after creating a selection container to label each container one by one. This PR makes the titles a proper tuple so titles can be set at creation time or all at once.

[jasongrout]

@pbugnion or @vidartf - any chance you could review this?

[pbugnion]

The interface is much better! Thank you!

The only thing that's a little strange is that we don't enforce that .children and titles is the same length. I'm guessing enforcing that they are the same length would make it hard for users to change the number of children: you'd have to hold trait notifications while you changed both. Not enforcing length consistency therefore seems sensible.

I think this is good to merge.