deskflick 0.7.1 — no more periodic pointer stutter

Fixes a periodic freeze of all mouse input. deskflick rescanned every input device and reopened the watched keyboards every two seconds, ~200ms of blocking work on the same thread that relays your mouse to the compositor. It now rescans only when devices actually change, and does that work off the event loop. Worst-case latency drops from 200ms to 2ms.