- Include Sets/Zones now scans immediately to refresh the list of available
  mesh parts. Unfortunately, this also causes the object panel to be
  modified, even although the selection doesn't acutally need to change.
  This seems to be due to how the pqNamedWidgets are getting the information
  from the proxy properties. I can't figure if it's possible to acheive what
  I want, but we can probably live with the current implementation.

  After IncludeZones, simply us 'Reset' to undo the spurious GUI changes.
  Works fine - just looks a bit silly.

- Added 'Refresh' button to rescan for new times/fields.  Good for
  post-processing ongoing calculations without exiting the reader.

- Added 'Skip Zero Time' checkbox: many (some) calculations have
  data missing at time=0 (eg, rho, lagrangian, etc). This provides
  a convenient way to skip over this time.

- Future?:
  We could probably pick up favourite default values for these switches from
  ~OpenFOAM/controlDict, from a case system/paraview, or simply by making
  the casename.OpenFOAM also be an OpenFOAM dictionary with the settings.