Completely overhauled Popovers.md with a comprehensive guide to popovers in Terminal.Gui. Added detailed explanations, requirements, and best practices for creating, registering, and managing popovers. Included extensive code examples for PopoverMenu, custom popovers, and common scenarios such as context menus and autocomplete. Documented keyboard and mouse event handling, lifecycle management, and provided an API reference. Removed outdated and unstructured content.
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