Sort actions by (priority, due date)
Highlight-ignore DONE and WONTDO in .vtd files
Missing Action stubs; Include-all by default; ...

New libvtd.

Major changes:
- Projects without NextActions will now show a stub.  User can decide whether to
  add a NextAction or check it off -- in the latter case, checking off the stub
  action checks off the Project!
- All NextActions are now shown by default.  Only if at least one context is
  "include"'d, do we restrict by context.  (Excluded contexts are still
  respected.)
- Visible dates now get inherited too.

Minor changes:
- Tweaked subunit regex for recurring actions so ' - ' is the separator.
- ProjectsWithoutNextActions now ignore DONE projects.