Add a dynamic keyboard shortcut help page

The page could use https://github.com/xwiki/xwiki-platform/blob/a5f837906cb6d6db8d28cfdcccd6b66b2cd30b4c/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-war/src/main/webapp/resources/js/xwiki/xwiki.js#L835 and should display all the shortcuts and their values in the current context. Some shortcuts are only available in edit mode, they should be displayed too. The values should adapt on the current space and the current user.

The dynamicity is important to handle the 2 use cases:

• Extensions or custom code can contribute keyboard shortcuts
• Users can remapped some shortcuts

Ideally as a popup dialog, which can be triggered from various places (with a keyboard shortcut , in the Help UI, in the wysiwyg editor, in the wiki editor).