Reversed PR #754; Added a way for ng-repeat to track by

Pull request https://github.com/stellar/stellar-client/pull/754 updates the paths only if something changed to try and solve https://github.com/stellar/stellar-client/issues/710.

This new PR solves the issue in a better way by using `track by` so that even if a path changed, the DOM element for sending a currency wouldn't be re-rendered.
Add a comment above this to explain what it is used for.