cleanup: Use Array.includes() to check for element existence

cleanup: Use Array.includes() to check for element existence

We can use that newer method where we don't care about the actual position
of an element inside the array.

(Array.includes() and Array.indexOf() do behave differently in edge cases,
for example in the handling of NaN, but those don't matter to us)

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/152