boxpointer: Swap arrow alignment on RTL

boxpointer: Swap arrow alignment on RTL

The current code uses the arrow alignment to align the menu between
the top/left (0) and bottom/right(1), regardless of the text direction.

Instead of adjusting code everywhere, reinterpret the parameter
to refer to start/end instead, and swap the alignment with an
RTL text direction.

Credit to Sebastian Keller for spotting the cause of the issue.

Close https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5261
Close https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7356

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3150>