idle_notify: debounce inactivity timers

[cmeissl]

when processing multiple input events we
can delay re-registering the timers until
all event have been processed.
this safes us from cancelling and registering
the timers multiple times.

[cmeissl]

@mayakwd I tried to further optimize this a bit, if you got some spare time I would appreciate if you can check if this lowers CPU usage for you.

[YaLTeR]

FWIW niri already debounces these on its side, without using extra idles:

https://github.com/niri-wm/niri/blob/8e3e93b6240dc9dbc702f0b44f20177c2ce737cf/src/niri.rs#L6050-L6053

https://github.com/niri-wm/niri/blob/8e3e93b6240dc9dbc702f0b44f20177c2ce737cf/src/niri.rs#L775-L776

[mayakwd]

@mayakwd I tried to further optimize this a bit, if you got some spare time I would appreciate if you can check if this lowers CPU usage for you.

Hey, sure. I'll check it in the evening.