The selected unit for an annotation type should not be remembered when creating a new annotation type

[Nicklas Nordborg]

To reproduce:

1. Create a numerical annotation type. Select any quantity and unit on the units tab. Save.
2. Create a second numerical annotation type. Do not switch to the units tab. Save.
3. The second annotation type is created with the same quantity as the first annotation.

The problem is that an annotation type that has been given a unit can't be made unit-less. A new annotation type must be created. On the other hand, a unit-less annotation type can be converted to an annotation type with a unit so it is always safer to not assign a unit automatically.

[Nicklas Nordborg]

(In [5152]) Fixes #1412: The selected unit for an annotation type should not be remembered when creating a new annotation type