Prototype an idea for nested label inputs.

Add a new label template and modify the existing templates to make it
easy (and the default) to have radio/checkboxes embedded in their
labels. Because bootstrap is pretty common this layout gets requested
pretty often for checkbox/radio inputs.

This still needs tests written/updated but I wanted to get some early
feedback on the idea and approach.
Rename nested to nesting.
Fix up tests for nested input/labels.

Fixup nested inputs and add a new test showing how you can update the
templates to have non-nested inputs.
Fix CS errors.
Add tests showing multicheckbox nesting.

Add tests to ensure that multi-checkbox can be un-nested using common
templates.
Merge pull request #4929 from cakephp/nest-label-2

Prototype an idea for nested label inputs.