The switcherPopups use _next() and _previous() to get the items
in the text direction. I. e. with LTR _next() gets the right item;
on RTL it gets the left item. This doesn't work well with RTL when using
the arrow keys since the text direction doesn't matter in those cases.
Pressing Left Arrow should still move left regardless of text direction.
So use the opposite methods.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2547
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1835>
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