not a generic way
it should add a real epson gadget

Keep compatibility with existing officejs tests, by keeping the
data-i18n attributes, even if useless.

Chrome will deprecate appcache in the coming weeks.
As appcache has been dropped on Firefox, this change will improve the speed on Firefox.

The service worker only provide a cache for all ERP5JS gadgets.
ERP5JS will still be usable on browser without service worker API.

The list of files to cache is directly added inside the service worker
code, in order to be able to skip the usage of the fetch method (which
does not yet cover all browser HTTP usage).

This list is configurable on the web site level.
No need to change the service worker code directly.

The service worker will be updated as soon as the web site modification date changes.

End user does not need to reload its browser tab.
Only browsing ERP5JS will trigger the service worker update.

When a new update is found, ERP5JS will automatically reload itself if
there is no risk of losing user data input.

An empty appcache is kept, to allow migration of existing client to the
worker automatically.

See nexedi/erp5@2e3f7a8a

Use the precache script to get the list of gadgets.
Ensure to also check the launcher gadget.

Report subgadget service error.

Replace it by domsugar.

This speed up gadget class generation (handlebars template parsing is
slow).

This attribute is only needed for the translateHTML method.
There is no reason for it to be kept in the final DOM HTML.

Add service worker precache script.

Add service worker precache script

renderJS now correctly handles relative URL.

This allow edit scripts or interactions to change the status message or
the status level using the same way as workflow scripts: script can set
the status text in request['portal_status_message'] and the status level
in request['portal_status_level'].

Note that part of this was already checked in accidentally in 97c40dcb