Support XmlHttpRequest via protocol message about tabris-js

[tbuschto]

My proposal:

notify "StateChange" {
    state: "headers" | "loading" | "finished" | "aborted" | "error" | "timeout"
    code: HTTP STATUS CODE,
    response: string
}
notify "Progress" {
    loaded: long,
    total: long
}

"StateChenge" covers XHR "load", "loadend", "abort", "error", "timeout" and state change 2-4
"Progress" covers "progress". Progress events are fired only while the state is "loading"
"loadstart" and state change 1 (OPENED) can be handled completely in JS (fired after a few pre-checks in the send/open methods, before delegating to the bridge.)

Perhaps "headers" and "loading" can be merged, I'm not sure if there is a state between having the headers but not yet a a single byte of the respose text.

[ralfstx]

Implemented in 49e55b4