[IMP] qweb doc, tests

* document qweb based (mostly) on JS version
* convert JS qweb tests to (mostly) language-independent XML so they can be
  used for JS and Python implementations
* add some more tests (e.g. precedence between t-value and body in t-set)
* remove ``t-import``
* fix parity in foreach(dict) (and rename some variables to make array and
  object versions more similar)