Restrict Tasks that can be required by the given Task

... restrict them to all valid (i.e. having non-empty name, excluding
the given Task) tasks of the current workflow. Hack Barricade.js a bit
to enable moving up the tree of containers in search of 'current
workflow'.

Change-Id: I16e6ac34d44aa2d4d921aaf9fd0c251362b7894f