[ADD] running of XML cases to python qweb
[ADD] qweb: handling of t-att=mapping

Changed render_att_att to return an iterable of pairs instead of a pair, and
dispatched t-att on whether its result is a Mapping.

Also changed qweb test runner so it uses ordereddict for JSON mapping in
params, otherwise iteration order (and thus order of attributes in output) is
unpredictable and results don't/can't match expectations (as both are
strings).

Note that this relies on JS implementation details wrt iteration order of
mappings. Tests would probably be somewhat less brittle if rendering output
was parsed to XML... if that's possible (?)
[REM] turns out escf and rawf had been knowingly removed and shouldn't be added back
[ADD] qweb: call directive's body

* __content__ can't be used in Python implementation because safe_eval, so use
  ``0`` from Python implementation instead
* remove postfix from t-call tests because due to implementation details all
  whitespace crap following a t-name is added to rendered template in Python
  impl, and don't want to normalize whitespace.
[IMP] qweb: foreach handling

* fix mapping handling to match JS impl: current value set as _value instead
  of being lost
* add handling of integer parameter
* only set _size and _last if current iterable is sized