Add _dumpViewport helper
[embedlite-components] Add asynchronous embedui:zoomToRect listener.

Send message through QuickMozView like:
sendAsyncMessage("embedui:zoomToRect", {"x": 10, "y": 200, "width": 200, "height": 300 })
Merge pull request #44 from rainemak/zoomToRect

[embedlite-components] Add asynchronous embedui:zoomToRect listener.