Correct labeling issue for field with non-alphabetic characters

The field names containing spaces or other special characters can not be used from the labeling drop-down menu. They can be selected, but the labeling fails silently.

This small patch encloses the field name with quotes when it contains any other characters than [a-zA-Z], allowing to use any field name. It does not add quotes for the most common case where a field is simply composed of letter.