BUG: units with greek letters or non-alphabetical chars aren't accepted by validator

When creating a spinbox with base_unit='us', for example, the string representation of the quantity will be '[value] μs'.

When the quantity string is converted to text on the lineEdit, the validator won't allow 'μs', because its regex only allows a-Z characters as units.

To fix this, we should add the support for characters like 'μ', 'Ω', 'º' that are used in units like microseconds, ohms, degree