Fix #9290 (crashes in symbology on fields with special characters)

Fixes several crashes and problems with backwards compatibility.
Before expressions were allowed, field names were loaded and saved
unquoted, e.g. x.y  - with introduction of expressions it was
necessary to use properly quoted "x.y" name so that expression
is correctly parsed. With these changes both options are possible
and when possible, the unquoted field name is used for compatibility