Add breadcrumb to engine layout

Following a discussion with @modsognir we decided to go with added a method to the engine's ``ApplicationController`` to return the category id that the engine would be considered inside of, for the purposes of the breadrumb.

That has been implemented, and a ``ToolCategory`` class introduced to wrap that and drive the breadcrumb.
Could try:

``it { is_expected.to_not be_home }``