Added fallback for hovering lookup on Linux systems

[PIPPOHSIOS]

As per requested, I added the fallback for hovering lookup with the shift key. So it should now work on Linux, though the window must be focused. However, I think that it would be more useful if we let the user decide whether they want the fallback or not, but I didn't add that feature so we can discuss it and see what works best for you. The pynput documentation states that if the script is running under X, it should work regardless of the sudo privileges. This way, if someone it's still using X, they might benefit from using the hovering lookup feature even when the window is unfocused.

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Cool, but don't check for Linux, rather check for wayland directly, like this:
os.environ.get("XDG_SESSION_TYPE") == "wayland"