[widget] Rename .in_node to .find_in.
Delete unused class.
[widget] Document a few methods.
[form] Delete class level .submit.

With the new #widget convention, this method is useless.
[form] Delete custom initialization.

The behavior can be replicated using other instance methods.
[widget] Have action method return the current widget.
[form] Define submit as an action.
[widget] Reword documentation.
[widget] Remove unnecessary argument from action macro.
[widget] Document the .action macro.
[widget] Make a widget's root node publicly available.
[widget] Remove dependency on the Conversions module.
[widget] Rename #fill_all to #set.
[widget] Move widget macros to another module.
[widget] Introduce .widget_delegator.
Bump to 0.3.1.