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Gadget does not take `undefined` arguments as a change to context.

This makes using of default state almost boiler-plate free. Allows to
easily avoid unnecessary changes by having `undefined` in default state.

For explicite zeroing the state attribute, one should use `null` value.

Iframe: ensure all channels are OK before calling ready

Do not update jslint version which deprecates indentation options.

Add onLoop service.
Allow manualBootstrap.

The inital loaded HTML does not contain anything of renderjs and gets added only after pageload.
Instead of registering init() for the DOMContentLoaded event directly, a deferred promise is used which gets resolve either by the event, or by a manual call of rJS.manualBootstrap().

Tests:
* injects rsvp/renderjs in a blank iframe
* makes sure renderjs is only present after it was injected
* calls manual bootstrap to initialzed renderjs
* creates small test gadget to ensure renderjs is working within the iframe (after bootstrap was executed)

onStateChanges bug fixes

Nearly drop iframe timeout.
Add reportGadgetDeclarationError acquired method.

This can be used to catched loading errors from HTML gadget declaration.
This will prevent the parent gadget to fail.

This should improve renderJS usage on slow network.

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 nexedi/renderjs!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