Add simple message sending via JS
Fix lines not being removed properly

layer_remove_child_layers does not work with scroll layers as we don't get a
reference to the content layer. Adding a layer multiple times isn't a problem
though, so the current solution works.
Implement receiving departure lines

Problem: Most messages don't come through properly.
Retry sending message until it succeeds

Some messages seem to take up to two retries though...
Send departures serially

This seems to stop messages from being dropped.