Document the new make_managed() & prefer to use it / Use auto more in examples [master]

This accompanies the main gtkmm merge requests at gtkmm#33 (closed)

This performs creation and manage()ment in a single step and therefore
avoids the user having to write the discouraged new operator, looks more
like Standard C++ things like make_shared(), etc. So, move our examples
to it, and elaborate on why it is preferable to manage() or new/delete.

I got sidetracked and tacked on a subsequent commit that uses auto a lot more, in the new code and elsewhere. I can split that if anyone want.