If you select a local variable, double-click on a reference entry,
and then hit "back", you aren't taken back to the correct place in
the local variable table. This is annoying if you're trying to
explore how a local variable is used.
The NavStack Location object now has a "line delta" that can be
applied to position the selection correctly. This isn't stable
across undo/redo, but it solves the common cases.
This makes LineListGen's "Top" class redundant, so uses of that have
been replaced with Location.
475c31b886