Add primary-selection filter

[notpeelz]

I'm working on a PR for niri to disable the primary selection. I've had to make some changes in Smithay in order to support disabling of the primary selection at runtime (live-reloading of the config).

Niri PR: YaLTeR/niri#1011

@kchibisov I was told you were the last person who reworked clipboard handling in Smithay, and that you might want to take a look at these changes.

[kchibisov]

If the filter is done the same way as any other filter, I'm not sure there's anything needed from my end, since it doesn't touch anything in clipboard handling itself.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 19.34%. Comparing base (953959e) to head (c64e0a7).

Files with missing lines	Patch %	Lines
src/wayland/selection/primary_selection/mod.rs	75.00%	4 Missing ⚠️
src/wayland/selection/wlr_data_control/mod.rs	20.00%	4 Missing ⚠️
src/wayland/selection/wlr_data_control/device.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1641      +/-   ##
==========================================
+ Coverage   17.33%   19.34%   +2.00%     
==========================================
  Files         171      171              
  Lines       28389    28401      +12     
==========================================
+ Hits         4922     5493     +571     
+ Misses      23467    22908     -559     

Flag	Coverage Δ
wlcs-buffer	16.84% <58.33%> (+0.02%)	⬆️
wlcs-core	16.58% <58.33%> (+0.02%)	⬆️
wlcs-output	6.84% <58.33%> (+0.02%)	⬆️
wlcs-pointer-input	18.31% <58.33%> (?)

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[Drakulix]

Thanks!