Feature/contacts extension #141
                
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During integration of 'object' and 'content' filter options to VCard requests, I did some required child and attribute enhancement to prop.js.
Later on I recognized that the required child and attribute functionality is more or less already in filter.js (but lacking namespace support).
Decided to merge filter.js and prop.js together into a common usable prop.js.
It's now used in the VCard request (for review). Could also be used for the other filtering stuff (which would make filter.js unnecessary), but would break backward compatibility due to the unified object names.
How to proceed?