backends/libinput: Add support for plugins

Vlad Zahorodniirequested to merge
work/zzag/libinput-plugins into master

The main purpose of libinput plugins is to help with unfixable things in libinput.

See https://who-t.blogspot.com/2025/05/libinput-and-lua-plugins.html for more details.

The KWIN_LIBINPUT_NO_PLUGINS=1 environment variable can be set to disable loading libinput plugins.

Milestone: TBD, at quick glance, it seems like a pretty safe thing to backport to 6.6.

Edited by Vlad Zahorodnii

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