Change Bake templates to be view based

This is a little rough around the edges, but aught to be good enough to now discuss.

The main functional benefit introduced here is that php tags in a bake template are treated as plain text. I.e. whereas before a template looked like this:

    echo "<?php\n"; ?>
    namespace <?= $namespace ?>\Controller\Component;

Now, by using a different tag pair to indicate template logic, it would look like this:

    <?php
    namespace <%= $namespace %>\Controller\Component;

I haven't fully converted the existing [template templates](https://github.com/AD7six/cakephp/tree/3.0-bake-view/src/Template/Bake/Template), apart from just cleaning them up there's the opportunity to move any of the ugly logic into a helper/element to simplify things (either in this PR or at any point in the future).
Theme makes sense given that template sets are now provided by plugins ~= themes.