Add EditorContextMenuPluginManager and refactor menu plugins

[KoBeWi]

Follow-up to #94582
It mostly refactors the code and improves the API a bit. Also filled/improved the documentation.

• added EditorContextMenuPluginManager class
• all context menu code was moved from EditorData to the NewClass
• replaced the 4 item callbacks with single one that takes Variant
• custom menu item IDs are now assigned when menu is created
• custom shortcuts call the callback directly instead of going instead of going roundabout way via IDs
• custom shortcuts now properly accept events and callbacks report errors
• duplicated slot enums are unified into single one in EditorContextMenuPlugin class

API changes (breaking!):

• added new add_context_menu_item_from_shortcut() method that reuses a previously added shortcut, so you don't need to repeat the callback
• remove_context_menu_plugin() no longer takes slot argument, only plugin
• CONTEXT_SLOT_SUBMENU_FILESYSTEM_CREATE renamed to CONTEXT_SLOT_FILESYSTEM_CREATE
• add_context_menu_plugin() uses the enum from EditorContextMenuPlugin

CC @citizenll

[akien-mga]

Thanks!