Optimize categories box

[michael_s]

Because the query to count the number of products can be resource intensive on a large site (see previous upload by 133t who by the way uses one wrong query in test.php skewing the results to be less favourable for this contribution) I added instructions to add a cache for that. This cache can be toggled and deleted in the admin but be used independently from the regular osC cache feature (so you can cache only this and not the categories, manufacturers and also purchased box).

For the "Hide products from customer groups for SPPC" where the regular osC cache could not be used because it is not specific for customer groups I added support for the customer group specific caching (with the exception of the manufacturers box, that not contains customer group specific queries). Also added are some bugfixes for this contribution (version 2.0.1 and below) that will be added to that contribution later (version 2.0.2).

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