🍒 tabbox: match Shift+Backtab against Shift+Tab

When users simultaneously press Shift and Tab, the keys are sometimes registered as Shift+Tab, and sometimes as Shift+Backtab. So we need to match received Shift+Tab against shortcuts containing Shift+Backtab, and vice versa. Previously the code only checks for the first case. This commit adds checks for the second case.

BUG: 438991 FIXED-IN: 6.0

(cherry picked from commit bc73e14e)