ENH: Add Units module

The unit module handle the unit in slicer. The units are singleton in the
scene that belong to a quantity. So far the only quantity available are
length and time. The units are settings saved with the application.

See Issue #1694
ENH: Add Unit compatible widget

Added new widgets that can observes the MRML scene to deduce the
current unit for a quantity type (the singleton vtkMRMLSelectionNode is
used for storage in this case).

See Issue #1694
ENH: Apply units widget to slicer

Modified:
- Volume info widget: Made it a qMRMLWidget so it can propagate the
mrml scene.
- Annotations: Add to vtk widget and GUI
- Tractography Interactive seeding: Add units to GUI
- Slice controller: add units in spinboxes
- Volumes: add units to GUI
- Editore: add units to paint and island effects

See Issue #1694