Update the symbol lookup in EditInstructionOperand, EditDataOperand,
and GotoBox to correctly deal with non-unique labels.
This is a little awkward because we're doing lookups by name on
a non-unique symbol, and must resolve the ambiguity. In the case of
an instruction operand that refers to an address this is pretty
straightforward. For partial bytes (LDA #>:foo) or data directives
(.DD1 :foo) we have to take a guess. We can probably make a more
informed guess than we currently are, e.g. the LDA case could find
the label that minimizes the adjustment, but I don't want to sink a
lot of time into this until I'm sure it'll be useful.
Data operands with multiple regions are something of a challenge,
but I'm not sure specifying a single symbol for multiple locations
is important.
The "goto" box just finds the match that's closest to the selection.
Unlike "find", it always grabs the closest, not the next one forward.
(Not sure if this is useful or confusing.)
