Performance improvements.

Allow browser to fetch the JS files in parallel.
Loading should stay synchronous.

Speed up loading the declarative sub gadget definition.

Use document fragment to add all dependencies with one DOM call only.

Do not create a DOM document to calculate an URL.

Resetting the internal gadget state leads to unexpecting errors, and so,
it was a bad idea.

In case of onStateChange cancellation/error, reset the internal gadget state.
It will simplify the code of each onStateChange function, as developpers will
not have to handle such issue.

@seb , please review

/reviewed-on !7

* add an internal gadget state
* rename __element property to element
* add declareJob function
* add onEvent method
* 'this' in ready is now the gadget instance InlineSide-by-side

Changes on this state should be done by calling the changeState method.

If modifications are detected, an onStateChange callback will be triggered,
with the modified state keys as parameter.

Gadget can now access their DOM element without using not needed asynchronous code.

Jobs are executed on demand only when the gadget is in the DOM.

Calling the same job a second time cancel the previous execution.

This will declare a service to listen to a DOM event on the gadget element.

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).