Rewrite the layout helpers to use kiwi.

This rewrite drastically simplifies the layout helper architecture.
Making it easier for users to follow should they wish to implement
their own custom helpers.
fix typos
update constrainable widgets for new layout helpers
remove redundant docstring
update docstring
fix validation rules and small bug in layout helpers
start backend converstion to kiwi
fix typos
minor updates
remove commented code
update usage of size hint guard
update the container layout handling
use zero size instead of invalid size for minimum
fix bug when no items present for layout
use an edit context for changing edit variables
connect resized handler after solver init
remove old layout manager code
start to refactor out common code
extend constrainable interface to include size hint strengths
more work on layout manager