Fix custom cache key support **API change needed**

The get_output_cache_keys() called the singular get_output_cache_key()
However, this causes incorrect results when overriding these functions.
To fix this API, both functions now use a common base.

The get_output_cache_key() should add the specific key for that item.
The get_output_cache_keys() should add all possible variations for that item.

The get_rendering_cache_key() no longer adds a language code of the
current item (it didn't in 0.9), as that makes it possible to add this
yourself when needed. Contentitems have a single language after all.