* [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.
