Fixes #3700. ContextMenu pollutes Toplevel.MenuBar's key bindings

[BDisp]

Fixes

• Fixes ContextMenu pollutes Toplevel.MenuBar's key bindings  #3700

Proposed Changes/Todos

• Remove static from the MenuItem _menuBar field
• Only populate the ContextMenu keybindings when call the Show method.
• Add #nullable enable to MenuBar, Menu, MenuBarItem, MenuItem and ContextMenu
• Adapt the Views and Scenarios to use the changed ContextMenu

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