- drop the gtkSignalHandlers map entirely (wasn't used)
- use 'uint' for mapping signal IDs to MenuItem
- store and retrieve the menuitem identifier to/from the menu item widget
Linux requires a `gtk_menu_bar` for a gtk_window to display a menu.
For the `systray` a `gtk_menu` is needed instead.
This change creates the correct type of `impl` for the `Menu`
depending on how it is being used.