refactor login box & form to enable easy pluggability
* vregistry.selectable: get all selectable object of fixed oid with given context
* template headeri, logbox, logform: reorganize a bit the structure
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared