fix: replace XWayland backend with Wayland in Wayland sessions

[Integral-Tech]

• Replace XWayland with Wayland in Wayland sessions
• Refactor bash script & pass arguments to timeshift-gtk

Closes: #387

[mtwebster]

Hi, please refactor due to 98af104

Thanks!

[SnaetWarre]

can we get this to prod?

[mtwebster]

#460 was merged, which makes this stuff a bit cleaner, maybe we can add WAYLAND_DISPLAY to copy_env() instead?

[Integral-Tech]

#460 was merged, which makes this stuff a bit cleaner, maybe we can add WAYLAND_DISPLAY to copy_env() instead?

Sorry for my late reply.
WAYLAND_DISPLAY need to be set to $XDG_RUNTIME_DIR/$WAYLAND_DISPLAY, otherwise Timeshift may fail to launch:

> pkexec env WAYLAND_DISPLAY="$WAYLAND_DISPLAY" XDG_RUNTIME_DIR=/run/user/0 timeshift-gtk
(timeshift-gtk:240870): Gtk-WARNING **: 22:33:53.550: cannot open display:

However, we can add GTK_THEME to copy_env() to ensure the theme remains consistent with other GTK apps.