Assetmanager instances

[HoneyHazard]

• SSAssetManager overhauled
• Implements a simple caching mechanism
• SSAssetItem removed
• Interaction with the class simplified, consists of using static functions and the Context class:
• Context class, used outside of SSAssetManager only when passed to constructors for classes that call Open(string filename) on it to get stream
• RegisterLoadDelegate(LoadDelegate dlg)
• GetInstance(SSAssetManager.Context context, string filename) or GetInstance(string context, string filename)
• DeleteInstance(Context context, string filename)
• AddAssetArchive(ISSAssetArchiveHandler handler)