Convert qweb (Python) to use lxml

The current implementation uses minidom, on a crawler run under profiler on my machine ~40s (out of ~280s) are spent just parsing QWeb templates. A call to `lxml.etree.fromstring` next to `xml.dom.minidom.parseString` in `load_document` is profiled to 1.6s.

So here's a tentative full conversion.