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)