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 #5261 (closed)