r858: Merge 2.1:

r858: Merge 2.1:
Add garbage collection and reference-counting to Asset and CICacheItem classes.
Additionally, the frame cache has been reworked considerably.

Added new GarbageObject and Garbage classes.  Classes being placed under
garbage collection inherit from GarbageObject, which provides reference
counting.  The Garbage class provides the Garbage::garbage singleton which
keeps track of all objects currently in use.
Garbage::garbage->delete_object() is the only acceptable way of deleting
a GarbageObject.

Asset and CICacheItem now inherit from GarbageObject (via multiple
inheritance).