[repository] #1460066: backport datafeed cube as cubicweb source
* add some attributes to CWSource to handle this kind of source
(not natural to put everything in 'config' string). Adding a CWSource
subclass has been attempted then rollbacked because it adds pain
to handle multi-sources planning and it introduce an ambiguity on
a generic relation (cw_source), which may be a penalty in multiple
case
* data feed sources are a new kind of source, namely 'copy based',
which have no effect on the query planner
* a data feed source is associated to a list of url and a parser (appobjects
in the 'parsers' registry
* entities imported by a data feed have cwuri set to their url on the distant
site, their cw_source relation point to the data feed source, though their
source stored in the entities table (returned by cw_metainformation) is their
physical source, hence 'system'