Static annotation for factory methods

[marcprux]

This PR adds a new @Static annotation indicating that a method is to be treated as static even when it isn't. This is in order to be able to use the Objective-C bindings from Scala companion objects, whose method implementations are not actually static. For example, the following can now be expressed in Scala:

  @Library("Foundation")
  @Name("NSMutableArray")
  object NSMutableArray extends ObjCClass {
    BridJ.register(classOf[NSMutableArray[_]])

    @Static @native def arrayWithCapacity(capacity: Int): Pointer[NSMutableArray[_]]
  }

  class NSMutableArray[T <: NSObject : ClassTag] extends NSArray[T] {
    @native def removeLastObject(): Unit
    @native def removeAllObjects(): Unit
  }