Add cache on Panels

Panels can contain code that execute queries to the database like for example the Tag Cloud. For a small instance with a few users and a few documents, loading data live is a great feature, but on big instances this could impact the performance of the server, as for one request you could end up having thousands of queries.

The proposal is to add a configurable option to enable cache in panels. By default, the cache system would be disabled for all panels in the standard distribution, but the users would have the option to enable cache in panels individually and choose at what level it should be applied : user / wiki ... (any level that makes sense).