Move client only code to a macro

[hexonaut]

Client-only code is currently handled by @clientInit meta data attached to a method. This is okay, but it leads to extra code required to invoke the function upon construction of the object on the client. IE

class MyObject {
    public function new () {
        //Generic setup

        #if js
        clientCode();
        #end
    }
    @clientInit
    public function clientCode ():Void {
        //Do something only on the client
    }
}

This should be switched to a macro that not only invokes the method upon load, but also if the constructor is called on the client.