[web] xml_escape CheckBox and Radio labels (closes #7672764)

[test/tox] Enforce installation of modified yapps 580a6821713d is not enough for tox apparently... The thing is that it will first install dependencies (the `deps` option from tox.ini), actually pulling yapps from Logilab github but then, it will install the project (that is, CubicWeb) from the sdist *along with its dependencies*, which will pull yapps again and override the previously installed version for some mysterious reason. So add a `pip install` command in tox.ini, so that the modified yapps is the last one installed.

[web] delete unused PropertySheet._sourcefile_mtime

[etwist] remove no-longer-necessary uicache cleaning It was done before we were actually up, which means it could be too eager when combined with #6893185. Closes #6893247.

[web] remove PropertySheet._cache Instead of remembering that a css file has been processed, we re-process it at every request. This prevents our cache getting out of date with the file system. Closes #6893185 While we're at it, write the file atomically so that we don't risk serving partial data.

[test] Add missing u'' Test actually works in py3k, but not in python 2... That has to be a first!

[data] Use correct syntax to unset CSS attributes (closes #8602446) 'default' is actually a recognized CSS cursor name (the regular pointer). Using this value doesn't revert to whatever cursor DOM elements have by default ('pointer' on links, 'text' on text, etc). The proper way to unset a CSS attribute on DOM elements is to set it to an empty string. http://stackoverflow.com/questions/2027935/how-to-remove-css-property-using-javascript

[server] install custom sql scripts before creating tables for the schema A cube might need something like 'CREATE EXTENSION postgis' before creating tables for its entities, to add new final types.

[hooks] update internal URL in doc string

[doc] Fix restructured tables + improve english a bit