Use better defaults for units

[quozl]

On start units default to Length - Cables.

This is not a useful unit, and distracts by introducing a unit that learners are unlikely to encounter.

The units should either;

• not default, i.e. to an empty ComboBox item,
• default to an appropriate localised value taken from the po/ files, or;
• default to metric system.

[quozl]

Units set to not default, by 7bb29e4.