* Expose `DefaultApplicationMenu`.
Add `FindByLabel` and `ItemAt` for finding menu items in a menu
* Add `Menu.RemoveMenuItem()`, `MneuItem.GetAccelerator()` and `MenuItem.RemoveAccelerator()`
* Remove `Update`
* Iterate when removing menu items
* Add `GetSubmenu()`
- 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.
