Expand set of things that work with double-click on opcode

If you double-click on the opcode of "JSR label", the code view
selection jumps to the label.  This now works for partial operands,
e.g. "LDA #<label".

Some changes to the find-label-offset code affected the cc65 "is it
a forward reference to a direct-page label" logic.  The regression
test now correctly identifies an instruction that refers to itself
as not being a forward reference.

SourceGen/DataAnalysis.cs

@@ -196,7 +196,8 @@ namespace SourceGen {
         }
 
         /// <summary>
-        /// Extracts the operand offset from a data item.
+        /// Extracts the operand offset from a data item.  Only useful for numeric/Address
+        /// and numeric/Symbol.
         /// </summary>
         /// <param name="proj">Project reference.</param>
         /// <param name="offset">Offset of data item.</param>