Caching of operation output is currently controlled by the
GeglOperationClass::no_cache field on a per-operation-class basis,
and by the GeglNode::dont-cache property on a per-node basis.
However, since these fields are boolean, they prevent us from
automating the decision of whether an op should be cached, or,
altenatively, they don't provide sufficient control over this
decision.

Instead, add a ternary GeglCachePolicy enum, which can be one of
AUTO, NEVER, or ALWAYS, and add a GeglOperationClass::cache_policy
field, and a GeglNode::cache-policy property, which use this type.
When the cache-policy property of a node in NEVER or ALWAYS, its
result isn't/is cached, respectively; when it's AUTO, the deicision
depends on the operation:  When the cache_policy field of the
operation class is NEVER or ALWAYS, the result isn't/is cached,
respectively; when it's AUTO, the result is cached only if the
operation implements get_cached_region().  Both...