Use `layer shell` instead of window if available

[akhilman]

Is your feature request related to a problem? Please describe.
A Niri tiling window manager does not display the SafeEyes window above the current screen. Instead, it places the window into a fullscreen column, triggering a scrolling animation.

Describe the solution you'd like
Instead of creating a regular window, SafeEyes should create a layer shell if supported, or include an option in the configuration file to manually enable this feature.

Describe alternatives you've considered
Currently, SafeEyes is mostly usable with Niri but does not lock the screen if triggered while the window manager is in overview mode.

Additional context
This issue was previously discussed in #700. Also there is discussion in Niri repo.

Here is sceenshot of SafeEyes that was activated while window manager being in overview mode:

[deltragon]

I've just found https://github.com/wmww/gtk4-layer-shell.
This project appears to be exactly what we would need for this - it works from Python, it allows us to keep the current GTK code and also cleanly fall back to the current window-based code.

Feel free to open a PR integrating this to the BreakScreenWindow. If I have time, I'll also look into this at some point.