Add a hook for plugins to add HTML code on pages
Modifying `Html::footer()` would allow plugins to add code on/as footer.
More method are impacted because some are dedicated to the help pages, modal/pop-up, etc.
I suggest adding hooks (say `$PLUGIN_HOOKS['add_code_in_header']` and `$PLUGIN_HOOKS['add_code_in_footer']`) for the plugin to allow more customization of GLPI general interface.
In my case, I am building a plugin to add snippets of plain HTML codes (disclaimers, notes, analytic trackers, etc.) on any pages.
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This has already been implemented in the upcoming GLPi 9.1.2