If a device is removed while the computer is hibernating, and then
reconnected after waking it up, Sway sends an “added” event without
first sending a “removed” event.
Yambar used to assert that an “added” event didn’t refer to an already
tracked device.
This patch changes this, and simply ignores duplicate “added” events.
Closes#177
This is already being done in the initial query response. Not doing it
in the input event handler too leads to an assertion if there are
multiple devices with the same ID.
Hopefully fixes#54
