Fix merging of widgets config in FormHelper constructor.

Relying on InstanceTrait::config() to merge the config caused problems as we
want the config array of each widget to be overwritten instead of being merged.

Also keeping the 'widgets' and 'registry' key in config served no purpose as
changing them at runtime wouldn't do anything since widget registry instance is
already created by through helper's constructor.

Fixes #5067
Fix typo in property name
More typos fixes
Merge pull request #5068 from cakephp/3.0-widget-merge

3.0 widgets merge