[AppDB] - speed loading of the main page by optimizing screenshot class

[Tony Lambregts]

Chris Morgan wrote:
> I think this is the cleanest this can be done until we get php5 on the server, 
> then we'll be able to private the images in the screenshot class.
>
> This patch defers loading the thumbnail and screenshot images into memory 
> until they are necessary.  Doing so requires that we not have any users of 
> the screenshot class accessing the $oScreenshotImage or $oThumbnailImage 
> fields directly.
>
>   
looks good