Refactoring post dependency handling to allow easier injection of dependencies.

This is not a complete pull request, but at the moment something in progress which in combination with #1526 shall implement possibilities to inject dependencies on post compilation and page/post list compilation (discussed in #1521).

This initial PR's commit provides the necessary functions to ```Post``` to allow to query all dependencies for a post (fragment) compilation or for a page/post list compilation, and the standard plugins and ```Nikola``` helper functions (```generic_page_renderer``` and ```generic_post_list_renderer```) are changed to include these additional dependencies. Also, ```Post```'s ```extra_deps()``` is not available anymore (only used internally); its only external use so far was in the ```task/posts.py``` plugin, which now uses ```Post```'s ```fragment_deps```, whose difference to ```extra_deps``` is that it returns some additional files which are already included in ```task.file_dep``` (and so ```update()``` will not add them again).

(So up to now, it's only refactoring. Next step is to add new functionality and move some code into the post compiler so the ```.dep``` file can be handled by it.)