ENH: Harmonize qMRMLNodeComboBox properties

Previously, the property currentNodeId of the node combobox was set with an
id (QString) and sent a signal with a node (vtkMRMLNode*) which can be
confusing in the widgets using it. Now, the node combobox as an id
property or a node property.
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.
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).
ENH: WIP: Apply units widget to slicer