Implement ext-idle-notify

[YaLTeR]

Taking over from #1021, cc @PolyMeilex @chrisduerr.

Changes:

1. Added notify_activity(&Seat), kept the WlSeat method under a different name.
2. Fixed a bug where uninhibiting didn't reset idle state resulting in two subsequent resumed events (forbidden by the protocol).
3. Added dead WlSeat data cleanup.
4. Some other minor things.

Impl in niri: YaLTeR/niri@main...idle-notify

[codecov-commenter]

Codecov Report

Attention: 177 lines in your changes are missing coverage. Please review.

Comparison is base (1074914) 21.48% compared to head (965921a) 21.32%.

Files	Patch %	Lines
src/wayland/idle_notify/mod.rs	0.00%	177 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1320      +/-   ##
==========================================
- Coverage   21.48%   21.32%   -0.17%     
==========================================
  Files         155      156       +1     
  Lines       24944    25121     +177     
==========================================
- Hits         5359     5356       -3     
- Misses      19585    19765     +180     

Flag	Coverage Δ
wlcs-buffer	18.41% <0.00%> (-0.14%)	⬇️
wlcs-core	18.09% <0.00%> (-0.13%)	⬇️
wlcs-output	7.75% <0.00%> (-0.06%)	⬇️
wlcs-pointer-input	20.17% <0.00%> (-0.16%)	⬇️

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[PolyMeilex]

My only concern with this code was lack of compositor implementation, but if we have niri code, I'm fine with it now.

LGTM, not going to merge it tho, as that would kinda be me reviewing my own code 😃

[PolyMeilex]

Thanks!