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/SourceGen.csproj

@@ -131,6 +131,7 @@
     <Compile Include="AsmGen\IGenerator.cs" />
     <Compile Include="AsmGen\LabelLocalizer.cs" />
     <Compile Include="AsmGen\StringGather.cs" />
+    <Compile Include="AutoLabel.cs" />
     <Compile Include="NavStack.cs" />
     <Compile Include="ChangeSet.cs" />
     <Compile Include="CodeAnalysis.cs" />