ENH: Only one unit per quantity accessible to the user

Now only one unit per quantity is accessible to the user. This unit can
be customized and it's logged in the settings so it can be restored
when opening the application.
On the implementation side of things:
- Using only one reference for the selection node. This allows to
access all the units node more easily.
- Remove the unit module and widget
- Creating the unit widget to easily customize units (add the plugins with
it)

See Issue #1694