Refactoring code to make it possible to nest a input inside a label
Adding test to show how to nest inputs inside labels
Added more docs for Form::label()
Replace public property View::$ext with protected View::$_ext.

The view file extension can longer be changed through controller.
Remove Controller::$ext.

To change view files extension you should now extend View class
and override the View::$_ext property.
Fix testcase.
Merge pull request #2969 from cakephp/3.0-nest-input

3.0 nest input
Merge pull request #2972 from ADmad/3.0-controller-view

3.0 Remove Controller::$ext