don't make the transition duration user-settable
Add a base TreeNode class

the first step in all Widgets supporting children
add a parent attribute to Widget
drop the TreeNode class
update docs
set the parent of the widget when adding to a container
no more need to track the dock parent
minor cleanup
add sizeLimits utility function
use new sizeLimits utility function
reorder code to avoid extra DOM reflow
add initial layout invalidation methods
wip