cleanup: Get rid of Lang.bind()
After replacing Lang.Class with ES6 classes and adopting arrow notation for anonymous callbacks, we only use the Lang module to bind `this` to named callbacks. However since ES5, this functionality is already provided by Function.prototype.bind() - in fact, Lang.bind() itself uses it when no extra arguments are specified. Just use the built-in function directly where possible, and use arrow notation in the few places where we pass additional arguments.
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