Changed children list to CTuple.

Also removed some unused refences in the widget code
and added support for tuples in the pack/unpack code.
use Tuple, not nonexistent CTuple
allow setting Tuple traitlet with list

matches traits behavior
s/CTuple/Tuple
Merge pull request #14 from minrk/cast-tuple

Cast from list in Tuple traitlet
don't check shape of ContainerWidget.children
remove incorrect is instance check in children_changed
Merge pull request #15 from minrk/ctuple

fix validation of ContainerWidget.children
don't validate ContainerWidget.children

just use a plain Tuple, so we don't have to define a new Trait
Merge pull request #16 from minrk/more-ctuple

don't validate ContainerWidget.children
Merge pull request #5221 from jdfreder/ctuple

Change widget children List to Tuple.