Fix default contexts logic

Previously, the default contexts would be reset every time we changed
the *type* of VTD View.  If we exclude 'work' by default, but comment
that line out in the "Contexts" view, the action of changing to the
"Next Actions" view would automatically exclude it again.
Clear ALL contexts if none supplied
Make vtd#view#ClearContexts() argument optional
Set ALL contexts to their default value

This seems like more consistent behaviour.