Improve error verbosity

Browser cancels all ajax requests after the beforepageunload event.
This leads to the global renderJS error handling, which is unpleasant from user point of view.

Altering base url in the iframe gadget resulted in loading resources not from
the gadget iframe url but from the root gadget url. This is a critical issue
when the iframe gadget comes from a different domain

It removes communication timeout with parent iFrame and solve the issue of timeout in case of a high load due to heavy JS processing
/reviewed-on nexedi/renderjs!2

Gadget can now be declared in HTML without any explicit scope.

It is necessary to use dropGadget to completely remove a gadget.

This sandbox download the gadget HTML code and convert it as a dataurl which is loaded in an iframe.
This may help to bypass web browser mixed content policy or CSP restrictions (like inline js).

When a gadget is in iframe, it checks its communication's channel by using timeout 100ms (the only way...).
If a timeout occurs, renderjs considers this gadget has no parent gadget and so, consider it as parent gadget by stopping acquisition.

A service is function which is executed outside the promise tree when the
gadget is attached to the DOM.
Services are stopped when the gadget is removed from the DOM.

You can declare a service with the "declareService" method, which takes a
function as parameter.

Service errors are reported to the parent gadget by acquisition on
"reportServiceError".
If no gadget catched the service error, the application will crash (document
body will display the raw error).