[web] use Separator "component" instead of deprecated BoxSeparator widget

Flake8 cubicweb/web/box.py

[test] Remove upper bound for cubicweb-localperms Version 0.4.0 does not use a deprecated API that we are planning to drop in forthcoming changesets. This requires changing expected values in unittest_cwconfig.py where we scan install cubes. * In CubicWebConfigurationTC.test_available_cubes(), localperms disappears from the list of expected cubes because this test method will look for new style cubes in located in cubicweb/test/data/libpython/ (we patch pkg_resources.iter_entry_points()) as well as old style cubes installed in $PREFIX/share/cubicweb/cubes; since the new cubicweb-localperms version is new-style, it won't be catched by this method. * In CubicWebConfigurationWithLegacyCubesTC.test_available_cubes(), localperms is still listed, but now with the package name, because this method will simply look at installed cubes whether old or new style.

[test] Make the expected value explicit in CubicWebConfigurationWithLegacyCubesTC.test_available_cubes() This will help with the next changeset, and avoiding computation in tests is a good thing in general.

[test] Inline _test_available_cubes() method in cwconfig tests Improves readability.

[DX] adding error messages to {ResultSet,ActionsRegistry}.possible_actions Try to make it more accessible, when we first encountered these errors it it didn't made any sense to us.