connect category systems to CPC

[moedn]

while using the IPC seems to be a great idea (#6) most (if not all) featured platforms (#9) will bring their own system (as e.g. OSHWA or keywords from OKHv1).

We could either

1. disable category search, as it's a mess anyway,
2. map systems onto each other as far as possible (and take IPC as system of reference)
3. embrace the mess and let people search for all conceivable categories in all conceivable systems resulting in highly inconsistent search results