Moved memory management and added C# code for new search types

- Memory management moved up into the SobekCM Engine library to restore caching of item Aggregations_Mgmt_AdminViewer (ultimately, will likely need separate memory caching classes for the engine and the UI?  Can't sit in the code due to references to SobekCM_Item.  Could seperate the SobekCM_Item repository model in the future as well though. )
- Added the C# classes and structures to support mime-type filtering on searches, currently supporting just 'Include resources with no images' ( SobekDigital WOLF-2 ).  Javascript to follow.
- Updated the main stored procedure for advanced searching to support an initial NOT ( i.e., only search element being MimeType NOT 'NONE' ).
- Corrected issue where ranking algorithm in advanced search would occasionally exclude items (especially for starting with NOT, but perhaps affected more).