Add raw evdev input emulation

[samvv]

This pull request adds another input emulation backend that uses evdev to create a virtual mouse/keyboard, bypassing Wayland restrictions. It is meant to be a fallback for the other Linux backends.

This backend successfully fixes input emulation issues on COSMIC and potentially other platforms that lack a proper remote desktop portal.

Limitations

Currently this feature requires lan-mouse to be run as root, e.g. with sudo -E lan-mouse. This is a temporary limitation that I hope to alleviate by creating a separate daemon and authentication mechanism.

To Do

• Discrete scrolling needs to be tested. I just put a formula I thought might work. If I'm correct this is a Mac feature, which I don't have.
• Natural scrolling doesn't work because presumably the WM is responsible for the conversion. We'd need to emulate it.
• A daemon that allows unprivileged execution of lan-mouse, as described above.

[samvv]

Potentially somewhat related: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2664