slapos_jio: Introduce annotated helper gadget

This gadget aims to centralise entire "helper" test on various contexts, in this way translate becomes easy and update information is quite simple.

In order to use the gadget, you should include a template on rjs_gadget_slapos_annotated_helper_html:

<script id="add-new-example-header-text" type="text/x-handlebars-template">
Some text or <details> <summary>Some test<summary> <p>Content...</p></details>
</script>

and include the gadget on the target HTML:

<div data-gadget-url="gadget_slapos_annotated_helper.html"
data-gadget-scope="annotated_helper"
data-template-id="add-new-example-header-text"
data-gadget-sandbox="public">
</div>

This is also supposed to be compatible with GadgetField or usable from getDeclaredGadget(...)