* [v3 linux/systray] dbus generation
* [v3 linux] systemtray dbus implementation
* [v3] add 'id' for MenuSeparator
This is needed in order to have a unique value for all
menuItem(s) such that the Linux implementation doesn't have to
generate new identifiers.
Allowing the reuse keeps a 1-1 mapping in place without any extra effort.
* [v3 example/systray] add radio group to example
* [v3 linux] stub out ExportStatusNotifierItem callbacks
Can only seem to get the `SecondaryActivate` to fire when doing a
3-finger click! I was expecting a right-click interaction to trigger it.
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.
