SourceGen creates "auto" labels when it finds a reference to an
address that doesn't have a label associated with it. The label for
address $1234 would be "L1234". This change allows the project to
specify alternative label naming conventions, annotating them with
information from the cross-reference data. For example, a subroutine
entry point (i.e. the target of a JSR) would be "S_1234". (The
underscore was added to avoid confusion when an annotation letter
is the same as a hex digit.)
Also, tweaked the way the preferred clipboard line format is stored
in the settings file (was an integer, now an enumeration string).
