Add explicit class to access the "x_doc_resources" cache

By having an own class representing this cache the construction of the
cache key can be done in a single place and the callers don't have to
worry about it.

Change-Id: I8bd5b9d7a47294237bf6028d98c338e68153ed89
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>