Remove deprecated method.

View::addScript() has been replaced by view blocks.
Use assertContains() instead of more verbose approach.
Add support for themes + cells.

Cells should inherit the current View's theme and use it correctly when
rendering.
Add missing template file.
Rename action to template.

`template` better fits how the property is used in practice.
Merge pull request #3404 from cakephp/cell-theme

3.0 - Make view cells work with themes