Add selectable auto-label styles

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).

SourceGen/DataAnalysis.cs

@@ -311,7 +311,7 @@ namespace SourceGen {
                 // incorporates the address.  It's possible through various means to end
                 // up with a user or platform label that matches an auto label, so we
                 // need to do some renaming in that case.  Shouldn't happen often.
-                Symbol sym = SymbolTable.GenerateUniqueForAddress(mAnattribs[targetOffset].Address,
+                Symbol sym = AutoLabel.GenerateUniqueForAddress(mAnattribs[targetOffset].Address,
                     mProject.SymbolTable, "L");
                 mAnattribs[targetOffset].Symbol = sym;
                 // This will throw if the symbol already exists.  That is the desired