Provide annotation for explicit definition of selector

[jokade]

We need to support an annotation for overriding the selector on a given extern method, e.g.

  @selector("foo:name:")
  def fooWithName(id: id, name: NSString): Unit = extern
  def foo(id: id, bar: NSString): Unit = extern

This is necessary to create bindings to methods that are identical in the method name and signature, but differ in the selector.