Add ApiGen support

apigen.css was taken from ApiGen's default style.css with 3 changes:
1. All images are inlined as data URIs
2. All styles are restricted to #content
3. The search bar, which doesn't work with Redmine embedded, is hidden

ApiGen is a tool similar to Doxygen but specifically designed for PHP. My company needs ApiGen support in order to integrate our ApiGen documentation into Redmine.