Added ability to use custom receive handler

[PanicMan]

Added ability to use custom receive handler to receive custom messages in own app, in this case all messages other than the download messages will be forwarded to own receive handler

[sarfata]

Thanks for sharing this. This is a nice addition but I am not sure we want to add this in the example at the moment, just to keep things as simple as possible.

I will keep your PR open and I will discuss this with my colleague to see what would be the best way to share that with everyone.

[PanicMan]

ok, cool! I think it's very useful as otherwise you are not able to receive any messages from JS...

[cpfair]

I did this the other way around: don't touch the netDL code, instead use the fact that app_message_set_x_handler returns the old callback to chain in additional ones (which defer to that old callback if the message is uninteresting). Much cleaner IMO.

[PanicMan]

Ok, that sounds interesting! Do you have an example to look at? I've seen nothing about that in the documentation...

[cpfair]

It's documented -
https://developer.getpebble.com/docs/c/Foundation/AppMessage/#app_message_register_inbox_received and could work something like this:

static AppMessageInboxReceived netdl_received_handler;
// ...
static void in_received_handler(DictionaryIterator *iter, void *ctx) {
  if (dict_find(iter, ...)) {
    // Do things
  } else {
    // Pass to netDL
    netdl_received_handler(iter, ctx);
  }
}
// ...
static void init(void) {
  netdownload_initialize(...);
  netdl_received_handler = app_message_register_inbox_received(in_received_handler);
}

[PanicMan]

Hmm, not bad, that could also be a solution for ntDL itself... What about the error handler? Is there also an better solution?