Added two buttons to add and remove files.
Files are stored in project inside a directory named "static",
itself located in svghmi CTN instance files. Exemple :
exemples/svghmi_csv_json_img_table/svghmi_0@svghmi/static/
Files are transfered as "extra_files", but with a prefix
added to their name to avoid conflics.
Overview: IDE and runtime(s)
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..
TODO: take content from https://hg.beremiz.org/beremiz_docs/